pinctrl: sirf: add missing DT-binding document

CSR SiRFprimaII pinmux controller

Required properties:

- compatible	: "sirf,prima2-pinctrl"

- reg		: Address range of the pinctrl registers

- interrupts    : Interrupts used by every GPIO group

- gpio-controller : Indicates this device is a GPIO controller

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

Please refer to pinctrl-bindings.txt in this directory for details of the common

pinctrl bindings used by client devices.

SiRFprimaII's pinmux nodes act as a container for an abitrary number of subnodes.

Each of these subnodes represents some desired configuration for a group of pins.

Required subnode-properties:

- sirf,pins : An array of strings. Each string contains the name of a group.

- sirf,function: A string containing the name of the function to mux to the

  group.

  Valid values for group and function names can be found from looking at the

  group and function arrays in driver files:

  drivers/pinctrl/pinctrl-sirf.c

For example, pinctrl might have subnodes like the following:

 uart2_pins_a: uart2@0 {

         uart {

                 sirf,pins = "uart2grp";

                 sirf,function = "uart2";

         };

 };

 uart2_noflow_pins_a: uart2@1 {

         uart {

                 sirf,pins = "uart2_nostreamctrlgrp";

                 sirf,function = "uart2_nostreamctrl";

         };

 };

For a specific board, if it wants to use uart2 without hardware flow control,

it can add the following to its board-specific .dts file.

uart2: uart@0xb0070000 {

	pinctrl-names = "default";

	pinctrl-0 = <&uart2_noflow_pins_a>;

}