Fix CPU_SUSPEND when invoked with affinity level higher than get_max_afflvl()

This patch fixes the assertion failure when CPU_SUSPEND is invoked with
an affinity level higher than supported by the platform by adding suitable
checks for affinity level within `psci_cpu_suspend`. Also added suitable
bound checks within `psci_aff_map_get_idx` to prevent indexing beyond array
limits.

Fixes ARM-software/tf-issues#260

Change-Id: I04b75c49729e6c6d1983add590f60146c8fc3630