This patch enables optional PSCI functions `PSCI_STAT_COUNT` and
`PSCI_STAT_RESIDENCY` for ARM standard platforms. The optional platform
API 'translate_power_state_by_mpidr()' is implemented for the Juno
platform. 'validate_power_state()' on Juno downgrades PSCI CPU_SUSPEND
requests for the system power level to the cluster power level.
Hence, it is not suitable for validating the 'power_state' parameter
passed in a PSCI_STAT_COUNT/RESIDENCY call.

Change-Id: I9548322676fa468d22912392f2325c2a9f96e4d2