Add unit tests for Pointer Authentication

Add unit tests to:
    Test access to the key registers.
    Use the pointer authentication instructions.
    Call psci version and check the EL3 pointer authentication keys
    aren't leaked.
    Make a tsp call and check the secure world keys aren't leaked.

Change-Id: Ic7940757e6f9fc905ccef8c035e0c22b47b35cd7
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>