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
diff --git a/plat/qemu/qemu_sbsa/platform.mk b/plat/qemu/qemu_sbsa/platform.mk
index f34c7e1..51832d0 100644
--- a/plat/qemu/qemu_sbsa/platform.mk
+++ b/plat/qemu/qemu_sbsa/platform.mk
@@ -71,6 +71,8 @@
 BL31_SOURCES		+=	lib/cpus/aarch64/aem_generic.S			\
 				lib/cpus/aarch64/cortex_a53.S			\
 				lib/cpus/aarch64/cortex_a57.S			\
+				lib/semihosting/semihosting.c			\
+				lib/semihosting/${ARCH}/semihosting_call.S	\
 				plat/common/plat_psci_common.c			\
 				${PLAT_QEMU_COMMON_PATH}/qemu_pm.c		\
 				${PLAT_QEMU_COMMON_PATH}/topology.c		\