dt-bindings: gpio: document bindings supported by gpio-mpc8xxx.c

* Freescale MPC512x/MPC8xxx GPIO controller

Required properties:

- compatible : Should be "fsl,<soc>-gpio"

  The following <soc>s are known to be supported:

    mpc5121, mpc5125, mpc8349, mpc8572, mpc8610, pq3, qoriq

- reg : Address and length of the register set for the device

- interrupts : Should be the port interrupt shared by all 32 pins.

- #gpio-cells : Should be two.  The first cell is the pin number and

  the second cell is used to specify the gpio polarity:

      0 = active high

      1 = active low

Example:

gpio0: gpio@1100 {

	compatible = "fsl,mpc5125-gpio";

	#gpio-cells = <2>;

	reg = <0x1100 0x080>;

	interrupts = <78 0x8>;

	status = "okay";

};