dt-bindings: add documentation for banks

   configurations by referring to the phandle of that pin configuration node.

4. The gpio controller must be describe in the pinctrl simple-bus.

For each bank the required properties are:

- compatible: "atmel,at91sam9x5-gpio" or "atmel,at91rm9200-gpio"

- reg: physical base address and length of the controller's registers

- interrupts: interrupt outputs from the controller

- interrupt-controller: marks the device node as an interrupt controller

- #interrupt-cells: should be 2; refer to ../interrupt-controller/interrupts.txt

  for more details.

- gpio-controller

- #gpio-cells: should be 2; the first cell is the GPIO number and the second

  cell specifies GPIO flags as defined in <dt-bindings/gpio/gpio.h>.

- clocks: bank clock

Examples:

pinctrl@fffff400 {

	compatible = "atmel,at91rm9200-pinctrl", "simple-bus";

	reg = <0xfffff400 0x600>;

	pioA: gpio@fffff400 {

		compatible = "atmel,at91sam9x5-gpio";

		reg = <0xfffff400 0x200>;

		interrupts = <2 IRQ_TYPE_LEVEL_HIGH 1>;

		#gpio-cells = <2>;

		gpio-controller;

		interrupt-controller;

		#interrupt-cells = <2>;

		clocks = <&pmc PMC_TYPE_PERIPHERAL 2>;

	};

	atmel,mux-mask = <

	      /*    A         B     */

	       0xffffffff 0xffc00c3b  /* pioA */