drivers/gpio: Altera soft IP GPIO driver device tree binding

Adds a new driver device tree binding for Altera soft GPIO IP
Signed-off-by: NTien Hock Loh <thloh@altera.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

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

+Altera GPIO controller bindings
+
+Required properties:
+- compatible:
+  - "altr,pio-1.0"
+- reg: Physical base address and length of the controller's registers.
+- #gpio-cells : Should be 2
+  - The first cell is the gpio offset number.
+  - The second cell is reserved and is currently unused.
+- gpio-controller : Marks the device node as a GPIO controller.
+- interrupt-controller: Mark the device node as an interrupt controller
+- #interrupt-cells : Should be 1. The interrupt type is fixed in the hardware.
+  - The first cell is the GPIO offset number within the GPIO controller.
+- interrupts: Specify the interrupt.
+- altr,interrupt-trigger: Specifies the interrupt trigger type the GPIO
+  hardware is synthesized. This field is required if the Altera GPIO controller
+  used has IRQ enabled as the interrupt type is not software controlled,
+  but hardware synthesized. Required if GPIO is used as an interrupt
+  controller. The value is defined in <dt-bindings/interrupt-controller/irq.h>
+  Only the following flags are supported:
+    IRQ_TYPE_EDGE_RISING
+    IRQ_TYPE_EDGE_FALLING
+    IRQ_TYPE_EDGE_BOTH
+    IRQ_TYPE_LEVEL_HIGH
+
+Optional properties:
+- altr,ngpio: Width of the GPIO bank. This defines how many pins the
+  GPIO device has. Ranges between 1-32. Optional and defaults to 32 if not
+  specified.
+
+Example:
+
+gpio_altr: gpio@0xff200000 {
+	compatible = "altr,pio-1.0";
+	reg = <0xff200000 0x10>;
+	interrupts = <0 45 4>;
+	altr,ngpio = <32>;
+	altr,interrupt-trigger = <IRQ_TYPE_EDGE_RISING>;
+	#gpio-cells = <2>;
+	gpio-controller;
+	#interrupt-cells = <1>;
+	interrupt-controller;
+};