feat(psci): add support for PSCI_SET_SUSPEND_MODE

This patch adds a PSCI_SET_SUSPEND_MODE handler that validates the
request per section 5.20.2 of the PSCI spec (DEN0022D.b), and updates
the suspend mode to the requested mode.

This is conditionally compiled into the build depending on the value of
the `PSCI_OS_INIT_MODE` build option.

Change-Id: Iebf65f5f7846aef6b8643ad6082db99b4dcc4bef
Signed-off-by: Wing Li <wingers@google.com>