mmc: dw_mmc: use devres functions in dw_mmc

Use managed device resource functions for easy handling.
This makes driver simpler in the routine of error and exit.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>