lib: psci: early suspend handler for platforms

lib: psci: early suspend handler for platforms

This patch adds an early suspend handler, that executes with
SMP and data cache enabled. This handler allows platforms to
perform any early actions during the CPU suspend entry sequence.

This handler is optional and platforms can choose to implement it
depending on their needs. The `pwr_domain_suspend` handler still
exists and platforms can keep on using it without any side effects.

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