Fix MMC access on Sabrelite

It appears that MMC access on the Sabrelite has been broken since
cdcaee95:

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 <martyn.welch@collabora.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>