arm: mvebu: helios4: add MMC to SPL DT

This allows SPL to load the main U-Boot image from MMC once DM_MMC is
enabled.

Signed-off-by: Dennis Gilmore <dennis@ausil.us>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>