fvp: Rework when platform actions are performed

This patch reworks FVP port's power management implementation to perform
platform actions only when the platform exported hook is invoked for the highest
affinity level to enter/exit the OFF state.

For example, during a CPU_OFF operation, fvp_affinst_off() is called twice: for
affinity level 0 and affinity level 1 (in that order). CPU specific operations
are deferred until the next invocation if it is determined through a call to
psci_get_max_phys_off_afflvl() that this is CPU is the last in the cluster.

Similarly, during power up if the CPU is the first in the cluster, both CPU and
cluster specific operations are performed when fvp_affinst_on_finish() is
invoked for affinity level 1. Earlier, they were done across the two invocations
of the handler.

Change-Id: I4288ed3ba1385db36a69cc2e598deb219f209b8a