ARM: dts: mx6: Disable the keypad in the dtsi files

When booting a board that does not have a keypad (such as imx6q-sabresd) the
following error is seen on boot:

imx-keypad 20b8000.kpp: OF: linux,keymap property not defined in /soc/aips-bus@02000000/kpp@020b8000
imx-keypad 20b8000.kpp: failed to build keymap
imx-keypad: probe of 20b8000.kpp failed with error -2

Let's disable the keypad functionality in the dtsi files and let each board dts
enable it when needed.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>