samsung: arndale: remove board_mmc_init function

Remove board_mmc_init function.
It will be probed with driver-model.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>