Xilinx plb/axi GPIO controller

Dual channel GPIO controller with configurable number of pins
(from 1 to 32 per channel). Every pin can be configured as
input/output/tristate. Both channels share the same global IRQ but
local interrupts can be enabled on channel basis.

Required properties:
- compatible : Should be "xlnx,xps-gpio-1.00.a"
- reg : Address and length of the register set for the device
- #gpio-cells : Should be two or three. The first cell is the pin number,
  The second cell is used to specify channel offset:
		0 - first channel
		8 - second channel
  The third cell is optional and used to specify flags. Use the macros
  defined in include/dt-bindings/gpio/gpio.h
- gpio-controller : Marks the device node as a GPIO controller.

Optional properties:
- clock-names : Should be "s_axi_aclk"
- clocks: Input clock specifier. Refer to common clock bindings.
- interrupts : Interrupt mapping for GPIO IRQ.
- interrupt-parent : Phandle for the interrupt controller that
  services interrupts for this device.
- xlnx,all-inputs : if n-th bit is setup, GPIO-n is input
- xlnx,dout-default : if n-th bit is 1, GPIO-n default value is 1
- xlnx,gpio-width : gpio width
- xlnx,tri-default : if n-th bit is 1, GPIO-n is in tristate mode
- xlnx,is-dual : if 1, controller also uses the second channel
- xlnx,all-inputs-2 : as above but for the second channel
- xlnx,dout-default-2 : as above but the second channel
- xlnx,gpio2-width : as above but for the second channel
- xlnx,tri-default-2 : as above but for the second channel
- xlnx,no-init : No initialisation at probe


Example:
gpio: gpio@40000000 {
	#gpio-cells = <2>;
	compatible = "xlnx,xps-gpio-1.00.a";
	gpio-controller ;
	clock-names = "s_axi_aclk";
	clocks = <&clkc 71>;
	interrupt-parent = <&microblaze_0_intc>;
	interrupts = < 6 2 >;
	reg = < 0x40000000 0x10000 >;
	xlnx,all-inputs = <0x0>;
	xlnx,all-inputs-2 = <0x0>;
	xlnx,dout-default = <0x0>;
	xlnx,dout-default-2 = <0x0>;
	xlnx,gpio-width = <0x2>;
	xlnx,gpio2-width = <0x2>;
	xlnx,interrupt-present = <0x1>;
	xlnx,is-dual = <0x1>;
	xlnx,tri-default = <0xffffffff>;
	xlnx,tri-default-2 = <0xffffffff>;
} ;

Example to demonstrate how reset-gpios property is used in drivers:

driver: driver@80000000 {
	compatible = "xlnx,driver";
	reset-gpios = <&gpio 0 0 GPIO_ACTIVE_LOW>; /* gpio phandle, gpio pin-number, channel offset, flag state */
	reg = <0x0 0x80000000 0x0 0x10000>;
};
