Add board_mmc_init(...) function for init mmc1 only

Since B&R boards uses only MMC-Controller #1, it only
wastes time if we initialize #0 first to see that there is nothing.

Cc: <trini@ti.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>