mmc: Introduce mmc_phys_to_bus()/mmc_bus_to_phys()

This will allow us to use DM variants of phys_to_bus()/bus_to_phys()
when relevant.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>