dm: mmc: Add an MMC uclass

Add basic support for MMC, providing a uclass which can set up an MMC
device. This allows MMC drivers to move to using driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index 79e6fee..4eab274 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -1761,8 +1761,10 @@
 	INIT_LIST_HEAD (&mmc_devices);
 	cur_dev_num = 0;
 
+#ifndef CONFIG_DM_MMC
 	if (board_mmc_init(bis) < 0)
 		cpu_mmc_init(bis);
+#endif
 
 #ifndef CONFIG_SPL_BUILD
 	print_mmc_devices(',');