A simple common binding for matrix-connected key boards. Currently targeted at
defining the keys in the scope of linux key codes since that is a stable and
standardized interface at this time.

Required properties:
- linux,keymap: an array of packed 1-cell entries containing the equivalent
  of row, column and linux key-code. The 32-bit big endian cell is packed
  as:
	row << 24 | column << 16 | key-code

Optional properties:
Properties for the number of rows and columns are optional because some
drivers will use fixed values for these.
- keypad,num-rows: Number of row lines connected to the keypad controller.
- keypad,num-columns: Number of column lines connected to the keypad
  controller.

Some users of this binding might choose to specify secondary keymaps for
cases where there is a modifier key such as a Fn key. Proposed names
for said properties are "linux,fn-keymap" or with another descriptive
word for the modifier other from "Fn".

Example:
	linux,keymap = < 0x00030012
			 0x0102003a >;
	keypad,num-rows = <2>;
	keypad,num-columns = <8>;
