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/include/dm/uclass-id.h b/include/dm/uclass-id.h
index a961648..cba7c0a 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -36,6 +36,7 @@
 	UCLASS_LED,		/* Light-emitting diode (LED) */
 	UCLASS_LPC,		/* x86 'low pin count' interface */
 	UCLASS_MASS_STORAGE,	/* Mass storage device */
+	UCLASS_MMC,		/* SD / MMC card or chip */
 	UCLASS_MOD_EXP,		/* RSA Mod Exp device */
 	UCLASS_PCH,		/* x86 platform controller hub */
 	UCLASS_PCI,		/* PCI bus */