MIPS: ath79: Enable OF serial ports in the default config

CONFIG_SERIAL_OF_PLATFORM is needed to get a working console on the OF
boards, enable it in the default config to get a working setup out of
the box.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-kernel@vger.kernel.org