ppu_v1: add dummy shutdown callback implementation

shutdown/reboot commands from OS triggers system power shutdown
routine and currently this routine calls the set_state API from
ppu_v1 driver for shutting down the cores/clusters if shutdown
API is not implemented. Platforms may hang in indefinite while
loop in set_state API and the shutdown/reboot request may never
complete.

This patch adds a dummy shutdown API returning FWK_SUCCESS so
that the shutdown/reboot process continues as the CPU context
is not crucial during shutdown/reboot. This function is open
for addition to accommodate a proper routine for shutdown.

Change-Id: I253fba6c92a225419b01fdb58735b9eee3f734b6
Signed-off-by: Manoj Kumar <manoj.kumar3@arm.com>