ARM: dts: imx6: add pm_power_off support for i.mx6 chips

All chips of i.mx6 can be powered off by programming SNVS.
For example :
On i.mx6q-sabresd board, PMIC_ON_REQ connect with external
pmic ON/OFF pin, that will cause the whole PMIC powered off
except VSNVS. And system can restart once PMIC_ON_REQ goes
high by push POWRER key.
Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/boot/dts/imx6qdl-sabresd.dtsi

@@ -531,6 +531,10 @@ &pwm1 {
 	status = "okay";
 };
 
+&snvs_poweroff {
+	status = "okay";
+};
+
 &ssi2 {
 	status = "okay";
 };

arch/arm/boot/dts/imx6qdl.dtsi

@@ -665,6 +665,12 @@ snvs-rtc-lp@34 {
 					interrupts = <0 19 IRQ_TYPE_LEVEL_HIGH>,
 						     <0 20 IRQ_TYPE_LEVEL_HIGH>;
 				};
+
+				snvs_poweroff: snvs-poweroff@38 {
+					compatible = "fsl,sec-v4.0-poweroff";
+					reg = <0x38 0x4>;
+					status = "disabled";
+				};
 			};
 
 			epit1: epit@020d0000 { /* EPIT1 */

arch/arm/boot/dts/imx6sl-evk.dts

@@ -580,6 +580,10 @@ &pwm1 {
 	status = "okay";
 };
 
+&snvs_poweroff {
+	status = "okay";
+};
+
 &ssi2 {
 	status = "okay";
 };

arch/arm/boot/dts/imx6sl.dtsi

@@ -574,6 +574,12 @@ snvs-rtc-lp@34 {
 					interrupts = <0 19 IRQ_TYPE_LEVEL_HIGH>,
 						     <0 20 IRQ_TYPE_LEVEL_HIGH>;
 				};
+
+				snvs_poweroff: snvs-poweroff@38 {
+					compatible = "fsl,sec-v4.0-poweroff";
+					reg = <0x38 0x4>;
+					status = "disabled";
+				};
 			};
 
 			epit1: epit@020d0000 {

arch/arm/boot/dts/imx6sx-sdb.dts

@@ -336,6 +336,10 @@ &pwm3 {
 	status = "okay";
 };
 
+&snvs_poweroff {
+	status = "okay";
+};
+
 &ssi2 {
 	status = "okay";
 };

arch/arm/boot/dts/imx6sx.dtsi

@@ -671,6 +671,12 @@ snvs-rtc-lp@34 {
 					reg = <0x34 0x58>;
 					interrupts = <GIC_SPI 19 IRQ_TYPE_LEVEL_HIGH>, <GIC_SPI 20 IRQ_TYPE_LEVEL_HIGH>;
 				};
+
+				snvs_poweroff: snvs-poweroff@38 {
+					compatible = "fsl,sec-v4.0-poweroff";
+					reg = <0x38 0x4>;
+					status = "disabled";
+				};
 			};
 
 			epit1: epit@020d0000 {