dt-bindings: timer: Add Milbeaut M10V timer description

Add DT bindings document for Milbeaut M10V timer.
Signed-off-by: Sugaya Taichi <sugaya.taichi@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/timer/socionext,milbeaut-timer.txt

+Milbeaut SoCs Timer Controller
+
+Required properties:
+
+- compatible : should be "socionext,milbeaut-timer".
+- reg : Specifies base physical address and size of the registers.
+- interrupts : The interrupt of the first timer.
+- clocks: phandle to the input clk.
+
+Example:
+
+timer {
+	compatible = "socionext,milbeaut-timer";
+	reg = <0x1e000050 0x20>
+	interrupts = <0 91 4>;
+	clocks = <&clk 4>;
+};