ARM: dts: imx6dl-yapp4: Enable the MPR121 touchkey controller on Hydra

Add the touch keyboard present on Hydra board. The controller
is connected only using I2C lines. The interrupt line is not
available hence we use the polling mode.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>