refactor(psci): move timestamp collection to psci_pwrdown_cpu

psci_pwrdown_cpu has two callers, both of which save timestamps meant to
measure how much time the cache maintenance operations take. Move the
timestamp collection inside to save on a bit of code duplication.

Change-Id: Ia2e7168faf7773d99b696cbdb6c98db7b58e31cf
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
3 files changed