Add a small workaround into the platform code which forces the SDMMC
into 8-bit mode (the default configuration for all socfpga platforms)
to work around breakage caused by missing patches in mainline which
switch the probing of SD/MMC to OF instead of static configuraiton.

The patches will hit mainline after the SPL series, so to avoid build
issues, add this small temporary workaround.

Signed-off-by: Marek Vasut <marex@denx.de>