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
Set SCR_EL3.RW correctly before exiting bl31_main

SCR_EL3.RW was not updated immediately before exiting bl31_main() and
running BL3-3. If a AArch32 Secure-EL1 Payload had just been
initialised, then the SCR_EL3.RW bit would be left indicating a
32-bit BL3-3, which may not be correct.

This patch explicitly sets SCR_EL3.RW appropriately based on the
provided SPSR_EL3 value for the BL3-3 image.

Fixes ARM-software/tf-issues#126

Change-Id: Ic7716fe8bc87e577c4bfaeb46702e88deedd9895
Merge pull request #83 from athoelke:at/tf-issues-126
Merge pull request #67 from achingupta:ag/psci_standby_bug_fix