Documentation: dt: add bindings for TI Wakeup M3 IPC device

Add the device tree bindings document for the TI Wakeup M3 IPC
device on AM33xx and AM43xx SoCs. These devices are used by the
TI wkup_m3_ipc driver, and contain the registers upon which the
IPC protocol to communicate with the Wakeup M3 processor is
implemented.
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

Documentation/devicetree/bindings/soc/ti/wkup_m3_ipc.txt

+Wakeup M3 IPC Driver
+=====================
+
+The TI AM33xx and AM43xx family of devices use a small Cortex M3 co-processor
+(commonly referred to as Wakeup M3 or CM3) to help with various low power tasks
+that cannot be controlled from the MPU, like suspend/resume and certain deep
+C-states for CPU Idle. Once the wkup_m3_ipc driver uses the wkup_m3_rproc driver
+to boot the wkup_m3, it handles communication with the CM3 using IPC registers
+present in the SoC's control module and a mailbox. The wkup_m3_ipc exposes an
+API to allow the SoC PM code to execute specific PM tasks.
+
+Wkup M3 Device Node:
+====================
+A wkup_m3_ipc device node is used to represent the IPC registers within an
+SoC.
+
+Required properties:
+--------------------
+- compatible:		Should be,
+				"ti,am3352-wkup-m3-ipc" for AM33xx SoCs
+				"ti,am4372-wkup-m3-ipc" for AM43xx SoCs
+- reg:			Contains the IPC register address space to communicate
+			with the Wakeup M3 processor
+- interrupts:		Contains the interrupt information for the wkup_m3
+			interrupt that signals the MPU.
+- ti,rproc:		phandle to the wkup_m3 rproc node so the IPC driver
+			can boot it.
+- mboxes:		phandles used by IPC framework to get correct mbox
+			channel for communication. Must point to appropriate
+			mbox_wkupm3 child node.
+
+Example:
+--------
+/* AM33xx */
+	l4_wkup: l4_wkup@44c00000 {
+		...
+
+		scm: scm@210000 {
+			compatible = "ti,am3-scm", "simple-bus";
+			reg = <0x210000 0x2000>;
+			#address-cells = <1>;
+			#size-cells = <1>;
+			ranges = <0 0x210000 0x2000>;
+
+			...
+
+			wkup_m3_ipc: wkup_m3_ipc@1324 {
+				compatible = "ti,am3352-wkup-m3-ipc";
+				reg = <0x1324 0x24>;
+				interrupts = <78>;
+				ti,rproc = <&wkup_m3>;
+				mboxes = <&mailbox &mbox_wkupm3>;
+			};
+
+			...
+		};
+	};