dm: mmc: spl: Use the legacy block interface in SPL

Bring this in for SPL so that we can use generic code for loading from
block devices.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/Makefile b/drivers/Makefile
index 696b3ac..99dd07f 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -37,6 +37,7 @@
 obj-$(CONFIG_OMAP_USB_PHY) += usb/phy/
 obj-$(CONFIG_SPL_SATA_SUPPORT) += block/
 obj-$(CONFIG_SPL_USB_HOST_SUPPORT) += block/
+obj-$(CONFIG_SPL_MMC_SUPPORT) += block/
 
 else