PSCI: Validate non secure entrypoint on ARM platforms

PSCI: Validate non secure entrypoint on ARM platforms

This patch implements the platform power managment handler to verify
non secure entrypoint for ARM platforms. The handler ensures that the
entry point specified by the normal world during CPU_SUSPEND, CPU_ON
or SYSTEM_SUSPEND PSCI API is a valid address within the non secure
DRAM.

Change-Id: I4795452df99f67a24682b22f0e0967175c1de429