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>
diff --git a/drivers/block/Makefile b/drivers/block/Makefile
index a5e7307..dea2c15 100644
--- a/drivers/block/Makefile
+++ b/drivers/block/Makefile
@@ -5,9 +5,9 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_BLK) += blk-uclass.o
+obj-$(CONFIG_$(SPL_)BLK) += blk-uclass.o
 
-ifndef CONFIG_BLK
+ifndef CONFIG_$(SPL_)BLK
 obj-y += blk_legacy.o
 endif