devicetree: document Ingenic SoC UART binding

Add binding documentation for the UARTs found in Ingenic SoCs.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10161/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/serial/ingenic,uart.txt

+* Ingenic SoC UART
+
+Required properties:
+- compatible : "ingenic,jz4740-uart" or "ingenic,jz4780-uart"
+- reg : offset and length of the register set for the device.
+- interrupts : should contain uart interrupt.
+- clocks : phandles to the module & baud clocks.
+- clock-names: tuple listing input clock names.
+	Required elements: "baud", "module"
+
+Example:
+
+uart0: serial@10030000 {
+	compatible = "ingenic,jz4740-uart";
+	reg = <0x10030000 0x100>;
+
+	interrupt-parent = <&intc>;
+	interrupts = <9>;
+
+	clocks = <&ext>, <&cgu JZ4740_CLK_UART0>;
+	clock-names = "baud", "module";
+};