GICv3: Document GICv3 save/restore helpers
GICv3: Document GICv3 save/restore helpers

Give hints on how to use the GICv3 save/restore helpers in the
implementation of the PSCI handlers.

Change-Id: I86de1c27417b64c7ce290974964ef97ff678f676
Signed-off-by: Douglas Raillard <689105a89352b3b1d4ffa5bfffd804efe641e037@arm.com>

lsigithub/arm-trusted-firmware_public,lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware