dm: mmc: Allow disabling driver model in SPL

At present if U-Boot proper uses driver model for MMC, then SPL has to
also. While this is desirable, it places a significant barrier to moving
to driver model in some cases. For example, with a space-constrained SPL
it may be necessary to enable CONFIG_SPL_OF_PLATDATA which involves
adjusting some drivers.

Add new SPL versions of the options for DM_MMC, DM_MMC_OPS and BLK. By
default these follow their non-SPL versions, but this can be changed by
boards which need it.

Signed-off-by: Simon Glass <sjg@chromium.org>