mmc: mvebu: convert to driver model

This is a straightforward conversion of the old, non-dm driver. It was
done in-place as the deadline for non-dm MMC has passed. Previous
commits ensured that no board depends on the old, non-dm variant. Tested
on a Kirkwood based board with eMMC.

Signed-off-by: Harm Berntsen <harm.berntsen@nedap.com>
Tested-by: Harm Berntsen <harm.berntsen@nedap.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
CC: Pantelis Antoniou <panto@antoniou-consulting.com>
CC: Stefan Roese <sr@denx.de>
CC: Gerald Kerma <drEagle@doukki.net>
CC: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>