The i.MX6SL EVK needs this driver in android fastboot support. Add
this driver to u-boot.

To use the driver, user must define:

CONFIG_MXC_KPD          Enable the driver
CONFIG_MXC_KEYMAPPING   Key mapping matrix
CONFIG_MXC_KPD_COLMAX   The column size of key mapping matrix
CONFIG_MXC_KPD_ROWMAX   The row size of the key mapping matrix

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 5096e572667ff41217deb4ba9b1bd15e93fa6b59)