drivers/gpio: gpio-nomadik: Provide documentation for Device Tree bindings

Nomadik GPIO controller

Required properties:

- compatible            : Should be "st,nomadik-gpio".

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

- interrupts            : The interrupt outputs from the controller.

- #gpio-cells           : Should be two:

                            The first cell is the pin number.

                            The second cell is used to specify optional parameters:

                              - bits[3:0] trigger type and level flags:

                                  1 = low-to-high edge triggered.

                                  2 = high-to-low edge triggered.

                                  4 = active high level-sensitive.

                                  8 = active low level-sensitive.

- gpio-controller       : Marks the device node as a GPIO controller.

- gpio-bank             : Specifies which bank a controller owns.

- st,supports-sleepmode : Specifies whether controller can sleep or not

Example:

                gpio1: gpio@8012e080 {

                        compatible = "st,nomadik-gpio";

                        reg =  <0x8012e080 0x80>;

                        interrupts = <0 120 0x4>;

                        #gpio-cells = <2>;

                        gpio-controller;

                        supports-sleepmode;

                        gpio-bank = <1>;

                };