Input: spear-keyboard - document DT bindings

* SPEAr keyboard controller

Required properties:

- compatible: "st,spear300-kbd"

Optional properties, in addition to those specified by the shared

matrix-keyboard bindings:

- autorepeat: bool: enables key autorepeat

- st,mode: keyboard mode: 0 - 9x9, 1 - 6x6, 2 - 2x2

Example:

kbd@fc400000 {

	compatible = "st,spear300-kbd";

	reg = <0xfc400000 0x100>;

	linux,keymap = < 0x00030012

			 0x0102003a >;

	autorepeat;

	st,mode = <0>;

};