gxbb: Implement PSCI_CPU_OFF

This works fine for CPU1-3, but it fails for CPU0, where it is simply
ignored and leaves CPU0 in a WFI loop.

Change-Id: I7d73683fdd894f2021d6a5bc2cce6cd03e18e633
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>