Input: spear-keyboard - document DT bindings

Signed-off-by: NViresh Kumar <viresh.kumar@st.com>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>

Documentation/devicetree/bindings/input/spear-keyboard.txt

+* 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>;
+};