s5p_goni: enable mmc0

Adds the board_mmc_init function and enable the mmc command

Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>