Input: davinci_keyscan - add device_enable method to platform data

Add a function pointer in the platform data of the DaVinci Keyscan driver
called device_enable, in order to perform board specific actions when
the device is initialized, like setup the PINMUX configuration.

Signed-off-by: Miguel Aguilar <miguel.aguilar@ridgerun.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>