mips: octeon: octeon_ebb7304_defconfig: Enable MMC support

Enable MMC support including the regulator support on Octeon EBB7304.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Aaron Williams <awilliams@marvell.com>
Cc: Chandrakala Chavva <cchavva@marvell.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>