ARM: dts: keystone-k2g: Add PMMC node to support TI-SCI protocol

Texas Instrument's System Control Interface (TI-SCI) Message Protocol
is implemented in Keystone 2 generation 66AK2G SoC with the PMMC entity.

Add the ti-sci node representing this 66AK2G PMMC module.
Signed-off-by: Nishanth Menon <nm@ti.com>
[s-anna@ti.com: add unit address to DT node]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>

arch/arm/boot/dts/keystone-k2g.dtsi

@@ -139,5 +139,19 @@ msgmgr: msgmgr@02a00000 {
 			interrupts = <GIC_SPI 324 IRQ_TYPE_LEVEL_HIGH>,
 				     <GIC_SPI 327 IRQ_TYPE_LEVEL_HIGH>;
 		};
+
+		pmmc: pmmc@02921c00 {
+			compatible = "ti,k2g-sci";
+			/*
+			 * In case of rare platforms that does not use k2g as
+			 * system master, use /delete-property/
+			 */
+			ti,system-reboot-controller;
+			mbox-names = "rx", "tx";
+			mboxes= <&msgmgr 5 2>,
+				<&msgmgr 0 0>;
+			reg-names = "debug_messages";
+			reg = <0x02921c00 0x400>;
+		};
 	};
 };