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. The first cell is the pin number and the
  second cell is used to specify optional parameters (currently unused).
- gpio-controller : Marks the device node as a GPIO controller.

Optional properties:
- interrupts : Interrupt mapping for GPIO IRQ.
- 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


Example:
gpio: gpio@40000000 {
	#gpio-cells = <2>;
	compatible = "xlnx,xps-gpio-1.00.a";
	gpio-controller ;
	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>;
} ;
