qemu: Implement qemu_system_off via semihosting.

This makes the PSCI SYSTEM_OFF call work on QEMU. It assumes that QEMU has
semihosting enabled, but that is already assumed by the image loader.

Signed-off-by: Andrew Walbran <qwandor@google.com>
Change-Id: I0fb7cf7909262b675c3143efeac07f4d60730b03
5 files changed