juno_system: Add PSU management for GPU

The GPU power supply can be removed when its power domain
is set to OFF to improve power consumption efficiency.

This patch adds the logic to remove and reapply the power
supply for the GPU based on pre-state and post-state
power domain notifications.

Change-Id: I34db8b15955c0f8e868b525cde65275dda2b7fa8
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>