sunxi: board: do MMC pinmux setup for DM_MMC builds

Enabling DM_MMC skips the call to mmc_pinmux_setup() in board.c, as this
is supposed to be handled by the MMC driver, using DT information.

However we don't have a pinctrl driver yet, but would still like to keep
the working pinmux setup for our MMC devices. So bring this particular
call back to the DM_MMC code flow.

When booting from either SD card or eMMC, the SPL does the setup for us,
but when booting from SPI or USB we must not skip this part.

Fixes, boot via FEL or SPI flash, where the SPL won't setup the pinmux

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Jagan Teki <jagan@openedev.com>
[jagan: add Fix details on commit message]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>