Documentation: add description of DT binding for the gpio-mvebu driver

Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Acked-by: NRob Herring <rob.herring@calxeda.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NJason Cooper <jason@lakedaemon.net>

Documentation/devicetree/bindings/gpio/gpio-mvebu.txt

+* Marvell EBU GPIO controller
+
+Required properties:
+
+- compatible : Should be "marvell,orion-gpio", "marvell,mv78200-gpio"
+  or "marvell,armadaxp-gpio". "marvell,orion-gpio" should be used for
+  Orion, Kirkwood, Dove, Discovery (except MV78200) and Armada
+  370. "marvell,mv78200-gpio" should be used for the Discovery
+  MV78200. "marvel,armadaxp-gpio" should be used for all Armada XP
+  SoCs (MV78230, MV78260, MV78460).
+
+- reg: Address and length of the register set for the device. Only one
+  entry is expected, except for the "marvell,armadaxp-gpio" variant
+  for which two entries are expected: one for the general registers,
+  one for the per-cpu registers.
+
+- interrupts: The list of interrupts that are used for all the pins
+  managed by this GPIO bank. There can be more than one interrupt
+  (example: 1 interrupt per 8 pins on Armada XP, which means 4
+  interrupts per bank of 32 GPIOs).
+
+- interrupt-controller: identifies the node as an interrupt controller
+
+- #interrupt-cells: specifies the number of cells needed to encode an
+  interrupt source. Should be two.
+  The first cell is the GPIO number.
+  The second cell is used to specify flags:
+    bits[3:0] trigger type and level flags:
+      1 = low-to-high edge triggered.
+      2 = high-to-low edge triggered.
+      4 = active high level-sensitive.
+      8 = active low level-sensitive.
+
+- gpio-controller: marks the device node as a gpio controller
+
+- ngpios: number of GPIOs this controller has
+
+- #gpio-cells: Should be two. The first cell is the pin number. The
+  second cell is reserved for flags, unused at the moment.
+
+Example:
+
+		gpio0: gpio@d0018100 {
+			compatible = "marvell,armadaxp-gpio";
+			reg = <0xd0018100 0x40>,
+			    <0xd0018800 0x30>;
+			ngpios = <32>;
+			gpio-controller;
+			#gpio-cells = <2>;
+			interrupt-controller;
+			#interrupt-cells = <2>;
+			interrupts = <16>, <17>, <18>, <19>;
+		};