imx: imx8qm: add system off support

Add system power off support for i.MX8QM,
when Linux kernel issues "poweroff" command,
TF-A will send command to inform system
controller to power off whole board according
to board design, tested on i.MX8QM MEK board.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
diff --git a/plat/imx/imx8qm/imx8qm_psci.c b/plat/imx/imx8qm/imx8qm_psci.c
index b9b794b..53b5ccd 100644
--- a/plat/imx/imx8qm/imx8qm_psci.c
+++ b/plat/imx/imx8qm/imx8qm_psci.c
@@ -94,6 +94,7 @@
 	.pwr_domain_on = imx_pwr_domain_on,
 	.pwr_domain_on_finish = imx_pwr_domain_on_finish,
 	.validate_ns_entrypoint = imx_validate_ns_entrypoint,
+	.system_off = imx_system_off,
 };
 
 int plat_setup_psci_ops(uintptr_t sec_entrypoint,