serial: max310x: Add bindings documentation for the MAX310X UARTs

This patch adds the devicetree documentation for the Maxim MAX310X UARTs.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/maxim,max310x.txt

+* Maxim MAX310X advanced Universal Asynchronous Receiver-Transmitter (UART)
+
+Required properties:
+- compatible: Should be one of the following:
+  - "maxim,max3107" for Maxim MAX3107,
+  - "maxim,max3108" for Maxim MAX3108,
+  - "maxim,max3109" for Maxim MAX3109,
+  - "maxim,max14830" for Maxim MAX14830.
+- reg: SPI chip select number.
+- interrupt-parent: The phandle for the interrupt controller that
+  services interrupts for this IC.
+- interrupts: Specifies the interrupt source of the parent interrupt
+  controller. The format of the interrupt specifier depends on the
+  parent interrupt controller.
+- clocks: phandle to the IC source clock.
+- clock-names: Should be "osc" if clock is an external crystal or
+  "xtal" if an external clock source is used.
+
+Optional properties:
+- gpio-controller: Marks the device node as a GPIO controller.
+- #gpio-cells: Should be two. The first cell is the GPIO number and
+  the second cell is used to specify the GPIO polarity:
+    0 = active high,
+    1 = active low.
+
+Example:
+	max14830: max14830@0 {
+		compatible = "maxim,max14830";
+		reg = <0>;
+		clocks = <&clk20m>;
+		clock-names = "osc";
+		interrupt-parent = <&gpio3>;
+		interrupts = <7 IRQ_TYPE_EDGE_FALLING>;
+		gpio-controller;
+		#gpio-cells = <2>;
+	};