refactor(plat/synquacer): update PSCI system_off handling

SynQuacer SoC contains a Cortex-M3 System Control Processor(SCP)
which manages system power.
This commit modifies the PSCI system_off handling to call SCMI,
same as other PSCI calls. System power-off is done by turing off
the ATX power supply through GPIO, this operation is transferred
to SCP.

Note that this commit modifies only the SCMI case, obsolete SCPI
implementation is not updated.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Change-Id: I6c1009e67cccd1eb5d14c338c3df9103d63709dd
diff --git a/plat/socionext/synquacer/drivers/scp/sq_scmi.c b/plat/socionext/synquacer/drivers/scp/sq_scmi.c
index e2013cc..0e99256 100644
--- a/plat/socionext/synquacer/drivers/scp/sq_scmi.c
+++ b/plat/socionext/synquacer/drivers/scp/sq_scmi.c
@@ -189,6 +189,11 @@
 /*
  * Helper function to reset the system via SCMI.
  */
+void __dead2 sq_scmi_sys_shutdown(void)
+{
+	sq_scmi_system_off(SCMI_SYS_PWR_SHUTDOWN);
+}
+
 void __dead2 sq_scmi_sys_reboot(void)
 {
 	sq_scmi_system_off(SCMI_SYS_PWR_COLD_RESET);