MIPS: Octeon: cavium_octeon_defconfig: Enable Octeon MMC

Enable the Octeon MMC driver in the defconfig.

Signed-off-by: Steven J. Hill <Steven.Hill@cavium.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>