mock_psu: Add asynchronous driver support

This commit allows elements of the `mock_psu` module to emulate
asynchronous operations.

Enabling asynchronous operations forces requests to the element to be
pended for later completion, and is useful for testing interfaces that
interact with `mock_psu` which need to support this functionality.

Change-Id: Ic52221c510de8da60501a163b3f7dcacef6b0a3b
Signed-off-by: Chris Kay <chris.kay@arm.com>