Documentation: dt: add bindings for TI Wakeup M3 processor

Add the device tree bindings document for the TI Wakeup M3 remote
processor devices on AM33xx and AM43xx SoCs. These devices are used
to offload low-level power management functionality, and are handled
by the wkup_m3 remoteproc driver.
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>

Documentation/devicetree/bindings/remoteproc/wkup_m3_rproc.txt

+TI Wakeup M3 Remoteproc 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. This CM3 processor requires a firmware
+binary to accomplish this. The wkup_m3 remoteproc driver handles the loading of
+the firmware and booting of the CM3.
+
+Wkup M3 Device Node:
+====================
+A wkup_m3 device node is used to represent the Wakeup M3 processor instance
+within the SoC. It is added as a child node of the parent interconnect bus
+(l4_wkup) through which it is accessible to the MPU.
+
+Required properties:
+--------------------
+- compatible:		Should be one of,
+				"ti,am3352-wkup-m3" for AM33xx SoCs
+				"ti,am4372-wkup-m3" for AM43xx SoCs
+- reg:			Should contain the address ranges for the two internal
+			memory regions, UMEM and DMEM. The parent node should
+			provide an appropriate ranges property for properly
+			translating these into bus addresses.
+- reg-names:		Contains the corresponding names for the two memory
+			regions. These should be named "umem" & "dmem".
+- ti,hwmods:		Name of the hwmod associated with the wkupm3 device.
+- ti,pm-firmware:	Name of firmware file to be used for loading and
+			booting the wkup_m3 remote processor.
+
+Example:
+--------
+/* AM33xx */
+ocp {
+	 l4_wkup: l4_wkup@44c00000 {
+		compatible = "am335-l4-wkup", "simple-bus";
+		ranges = <0 0x44c00000 0x400000>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+
+		wkup_m3: wkup_m3@100000 {
+			compatible = "ti,am3352-wkup-m3";
+			reg = <0x100000 0x4000>,
+			      <0x180000 0x2000>;
+			reg-names = "umem", "dmem";
+			ti,hwmods = "wkup_m3";
+			ti,pm-firmware = "am335x-pm-firmware.elf";
+		};
+	};
+
+	...
+};