feat(psci): introduce 'pwr_domain_off_early' hook

feat(psci): introduce 'pwr_domain_off_early' hook

This patch introduces the 'pwr_domain_off_early'  hook for
platforms wanting to perform housekeeping steps before the
PSCI framework starts the CPU power off sequence. Platforms
might also want to use ths opportunity to ensure that the
CPU off sequence can proceed.

The PSCI framework expects a return code of PSCI_E_DENIED,
if the platform wants to halt the CPU off sequence.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: I6980e84fc4d6cb80537a178d0d3d26fb28a13853