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_gicv2.c b/plat/qemu/common/qemu_gicv2.c
index fb56622..2c358ea 100644
--- a/plat/qemu/common/qemu_gicv2.c
+++ b/plat/qemu/common/qemu_gicv2.c
@@ -37,3 +37,8 @@
 	/* Enable the gic cpu interface */
 	gicv2_cpuif_enable();
 }
+
+void qemu_pwr_gic_off(void)
+{
+	gicv2_cpuif_disable();
+}