mmc: Remove ops from struct mmc and put in mmc_ops

Remove the in-structure ops and put them in mmc_ops with
a constant pointer to it.

This makes the mmc structure smaller as well as conserving
code space (in theory).

All in-tree drivers are converted as well; this is done in a
single patch in order to not break git bisect.

Changes since V1:
Fix compilation b0rked issue on omap platforms where OMAP_GPIO was
not set.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>