Fix broken standby state implementation in PSCI

This patch fixes the broken support for entry into standby states
introduced under commit-id 'd118f9f864' (tf-issues#94). Upon exit from
the platform defined standby state instead of returning to the caller
of the SMC, execution would get stuck in the wfi instruction meant for
entering a power down state. This patch ensures that exit from a
standby state and entry into a power down state do not interfere with
each other.

Fixes ARM-software/tf-issues#154

Change-Id: I56e5df353368e44d6eefc94ffedefe21929f5cfe