qemu: Implement PSCI_CPU_OFF.

This is based on the rpi implementation from
https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/2746.

Signed-off-by: Andrew Walbran <qwandor@google.com>
Change-Id: I5fe324fcd9d5e232091e01267ea12147c46bc9c1
7 files changed