test(pmu): test that the PMU state is preserved accros a world switch

EL3 saves some PMU registers and changes others on SMC entry. This test
checks that regardless of what EL3 decides to do, we are left with the
same state we started with.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I6d69a30b0d87cdd7e442359cb87a62ffcbb3f000