ARM: dts: imx6: RIoTboard provide standby on power off option

This board, as well as some other boards with i.MX6 and a PMIC, uses a
"PMIC_STBY_REQ" line to notify the PMIC about a state change.
The PMIC is programmed for a specific state change before triggering the
line.
In this case, PMIC_STBY_REQ can be used for stand by, sleep
and power off modes.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6dl-riotboard.dts

@@ -84,6 +84,10 @@ &audmux {
 	status = "okay";
 };
 
+&clks {
+	fsl,pmic-stby-poweroff;
+};
+
 &fec {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_enet>;
@@ -164,6 +168,7 @@ pmic: pf0100@8 {
 		reg = <0x08>;
 		interrupt-parent = <&gpio5>;
 		interrupts = <16 8>;
+		fsl,pmic-stby-poweroff;
 
 		regulators {
 			reg_vddcore: sw1ab {				/* VDDARM_IN */