Try to leak counter values from secure world.

This patch introduces a series of tests that try to leak PMU counter values
from EL3 and S_EL1.

PMU events used:
	- CPU cycles via PMU counter PMCCNTR_EL0
	- Retired writes to PC via PMU counter PMEVCNTR0_EL0

This AARCH64-specific patch is for security fix:
https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/1789



The AARCH32 versions of these tests will be in a future patch.

Signed-off-by: Petre-Ionut Tudor <petre-ionut.tudor@arm.com>
Change-Id: Ib27948edadde30272e59a9ab208543703fa078bd