### 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
if default is not None and required:
            
            raise ValueError("internal error: required and default are mutually exclusive for %s" % k)
        if aliases is None:
            continue
        if not is_iterable(aliases) or isinstance(aliases, (binary_type, text_type)):
            raise TypeError('internal error: aliases must be a list or tuple')
        for alias in aliases:
            aliases_results[alias] = k
            if alias in parameters:
                if k in parameters and alias_warnings is not None:
                    alias_warnings.append((k, alias))
                parameters[k] = parameters[alias]
    return aliases_results
def _list_deprecations(argument_spec, parameters, prefix=''):
    """Return a list of deprecations
    :arg argument_spec: An argument spec dictionary
    :arg parameters: Dictionary of parameters
    :returns: List of dictionaries containing a message and version in which
        the deprecated parameter will be removed, or an empty list.
    :Example return:
    .. code-block:: python
        [
            {
                'msg': "Param 'deptest' is deprecated. See the module docs for more information",
                'version': '2.9'
            }
        ]
    """
    deprecations = []