MMC: add erase function to both mmc and sd

Erase is a very basic function since the begin of sd specification is
announced. Although we could write a bulk of full 0xff memory to the
range to take place of erase, it is more convenient and safe to
implement the erase function itself.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>