ARM i.MX51 babbage: Add keypad support

The keypad is on the accessory board of i.MX51 babbage
main board and is driven by Keypad Port(KPP) in SoC.
The keymap is the same to i.MX25 3stack platform as
the accessory board schematic tells that it is designed
in this way.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>