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
diff --git a/plat/qemu/common/qemu_private.h b/plat/qemu/common/qemu_private.h
index 71ea4de..4dc62f5 100644
--- a/plat/qemu/common/qemu_private.h
+++ b/plat/qemu/common/qemu_private.h
@@ -32,5 +32,6 @@
 
 void plat_qemu_gic_init(void);
 void qemu_pwr_gic_on_finish(void);
+void qemu_pwr_gic_off(void);
 
 #endif /* QEMU_PRIVATE_H */