Introduce test to access pointer authentication register

EL3 runtime firmware currently enables unconditional access to pointer
authentication registers from lower EL [1]. The test performs a read
access on a pointer authentication system register to ensure that the
access is permitted from a lower EL, and doesn't result in a trap to
EL3.

[1] https://github.com/ARM-software/arm-trusted-firmware/commit/3ff4aaaca44b75504aec5ab5b72cd587a6fcd432



Change-Id: I893604ebcd9e5df830d97cce405c2a7518c0b23c
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>