system_power: Use PPU shutdown handlers

System_power module uses drivers that are defined by
the power_domain module. These drivers can optionally
implement the shutdown function which is called during
the shutdown procedure.
This patch extends the shutdown procedure within this
module so the shutdown function in the drivers will be
called, if provided.

Change-Id: I8c02809ee5669f3651a19fa7b69a9c084521749e
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>