mips: octeon: octeon_ebb7304_defconfig: Enable USB storage support

This patch enables USB storage support with the necessary partition
support on the MIPS Octeon EBB7304.

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