mmc: mmc header fix

Structure definition used type block_dev_desc_t, defined in part.h, which
wasn't included in mmc.h. It worked only in circumstances when common.h,
or another header using part.h was incuded in implementation files.

Change-Id: I5b203928b689887e3e78beb00a378955e0553eb7
Signed-off-by: Mateusz Zalega <m.zalega@samsung.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>