mmc: tmio: Share register access functions

Move register access functions into a shared header.
Use sd_ctrl_write16 in tmio_mmc_dma.c:tmio_mmc_enable_dma().

Other than avoiding (trivial) open-coding, the motivation for
this is to allow platform-hooks in access functions to
be applied across all applicable accesses.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>