feat(pmu): add PMU support for Realms

This patch adds support for using PMU in Realms.
It adds 'bool pmu_enabled' and 'unsigned int pmu_num_cnts'
variables in 'struct rd' and 'struct rec.realm_info'.

Signed-off-by: AlexeiFedorov <Alexei.Fedorov@arm.com>
Change-Id: I13aad600a0215ba66d25be12ede5f4b86e6b018a