dm: blk: Enable CONFIG_BLK if DM_MMC is enabled

To speed up conversion to CONFIG_BLK, enable it by default when DM_MMC is
enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_noblk_defconfig b/configs/sandbox_noblk_defconfig
index 85610ed..bc5793f 100644
--- a/configs/sandbox_noblk_defconfig
+++ b/configs/sandbox_noblk_defconfig
@@ -68,6 +68,7 @@
 CONFIG_DEBUG_DEVRES=y
 CONFIG_ADC=y
 CONFIG_ADC_SANDBOX=y
+# CONFIG_BLK is not set
 CONFIG_CLK=y
 CONFIG_CPU=y
 CONFIG_DM_DEMO=y