ARM: riotboard: Enable OF_CONTROL and DM gpio/pin control

Enable OF_CONTROL and DM for gpio and pin control support
on the i.MX6D based riotboard.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Cc: "Eric Bénard" <eric@eukrea.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>