### Summary

An exception is thrown when an argspec expecting a dict with suboptions is given a string.
This only occurs when the argument in the argspec has suboptions.

### Issue Type

Bug Report

### Component Name

module_utils/common/parameters.py

### Ansible Version

```console
ansible [core 2.11.4]
```


### Configuration

```console
$ ansible-config dump --only-changed
```


### OS / Environment

RHEL 8

### Steps to Reproduce

`meta/argument_specs.yml`
```yaml
---
argument_specs:
  main:
    options:
      dict_option:
        type: dict
        options:
          some_option:
            type: str
```
Playbook:
```
---
- hosts: localhost
  gather_facts: false
  vars:
    dict_option: this is a string
  roles:
    - test
```

### Expected Results

An exception should not be thrown.

### Actual Results

```console
The full traceback is:
Traceback (most recent call last):
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/executor/task_executor.py", line 158, in run
    res = self._execute()
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/executor/task_executor.py", line 582, in _execute
    result = self._handler.run(task_vars=variables)
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/plugins/action/validate_argument_spec.py", line 83, in run
    validation_result = validator.validate(provided_arguments)
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/module_utils/common/arg_spec.py", line 244, in validate
    _validate_sub_spec(self.argument_spec, result._validated_parameters,
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/module_utils/common/parameters.py", line 761, in _validate_sub_spec
    unsupported_parameters.update(_get_unsupported_parameters(sub_spec, sub_parameters, legal_inputs, options_context))
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/module_utils/common/parameters.py", line 177, in _get_unsupported_parameters
    for k in parameters.keys():
AttributeError: 'str' object has no attribute 'keys'
fatal: [localhost]: FAILED! => {
    "msg": "Unexpected failure during module execution.",
    "stdout": ""
}
```


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
"""Process aliases from an argument_spec including warnings and deprecations.
    Modify ``parameters`` by adding a new key for each alias with the supplied
    value from ``parameters``.
    If a list is provided to the alias_warnings parameter, it will be filled with tuples
    (option, alias) in every case where both an option and its alias are specified.
    If a list is provided to alias_deprecations, it will be populated with dictionaries,
    each containing deprecation information for each alias found in argument_spec.
    :param argument_spec: Dictionary of parameters, their type, and valid values.
    :type argument_spec: dict
    :param parameters: Dictionary of parameters.
    :type parameters: dict
    :param alias_warnings:
    :type alias_warnings: list
    :param alias_deprecations:
    :type alias_deprecations: list
    """
    aliases_results = {}  
    for (k, v) in argument_spec.items():
        aliases = v.get('aliases', None)
        default = v.get('default', None)
        required = v.get('required', False)
        if alias_deprecations is not None:
            for alias in argument_spec[k].get('deprecated_aliases', []):
                if alias.get('name') in parameters:
                    alias_deprecations.append(alias)