boards: mma-200: change STDIO uart port.
boards: mma-200: change STDIO uart port.

Signed-off-by: Takeyoshi Kikuchi <af72ecfe6d0bf60ad0be9363518c74b6bed5ff74@centurysys.co.jp>
(cherry picked from commit e96ae0d8b2baa5fc35f29f26775b026dc66262e7)

centurysys/RIOT,centurysys/RIOT,centurysys/RIOT,centurysys/RIOT,centurysys/RIOT,centurysys/RIOT