refactor(psci): extract cm_prepare_el3_exit_ns() to a common location

PSCI on and suspend wakeup both end with a cm_prepare_el3_exit_ns() call.
Since they are equivalent to the caller, move the call to just after the
*_finish calls to deduplicate it.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I05c16dc6613aba357d20cc39cc43aab803d675e0
3 files changed