gpio: Add generic serializer DT binding

Add binding for generic parallel-in/serial-out shift register devices
used as GPIO.
Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
[Clarified ngpios semantic]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

+Generic Parallel-in/Serial-out Shift Register GPIO Driver
+
+This binding describes generic parallel-in/serial-out shift register
+devices that can be used for GPI (General Purpose Input). This includes
+SN74165 serial-out shift registers and the SN65HVS88x series of
+industrial serializers.
+
+Required properties:
+ - compatible		: Should be "pisosr-gpio".
+ - gpio-controller	: Marks the device node as a GPIO controller.
+ - #gpio-cells		: Should be two. For consumer use see gpio.txt.
+
+Optional properties:
+ - ngpios		: Number of used GPIO lines (0..n-1), default is 8.
+ - load-gpios		: GPIO pin specifier attached to load enable, this
+			  pin is pulsed before reading from the device to
+			  load input pin values into the the device.
+
+For other required and optional properties of SPI slave
+nodes please refer to ../spi/spi-bus.txt.
+
+Example:
+
+	gpio@0 {
+		compatible = "ti,sn65hvs882", "pisosr-gpio";
+		gpio-controller;
+		#gpio-cells = <2>;
+
+		load-gpios = <&gpio2 23 GPIO_ACTIVE_LOW>;
+
+		reg = <0>;
+		spi-max-frequency = <1000000>;
+		spi-cpol;
+	};