Fix MMC access on Sabrelite

Fix MMC access on Sabrelite

It appears that MMC access on the Sabrelite has been broken since
cdcaee9518:

Loading Environment from MMC... Card did not respond to voltage select!
*** Warning - No block device, using default environment

Remove the board_mmc_init() and related entries now that we should be
using DM_MMC, add PINCTRL so that things work as expected.

Signed-off-by: Martyn Welch <[email protected]>
Reviewed-by: Jaehoon Chung <[email protected]>
Reviewed-by: Fabio Estevam <[email protected]>
Tested-by: Troy Kisky <[email protected]>
Acked-by: Troy Kisky <[email protected]>