psci: utility api to invoke stop for other cores

psci: utility api to invoke stop for other cores

The API can be used to invoke a 'stop_func' callback for all
other cores from any initiating core. Optionally it can also
wait for other cores to power down. There may be various use
of such API by platform. Ex: Platform may use this to power
down all other cores from a crashed core.

Signed-off-by: Sandeep Tripathy <sandeep.tripathy@broadcom.com>
Change-Id: I4f9dc8a38d419f299c021535d5f1bcc6883106f9