[MMC] Add MMC class devices

Create a mmc_host class to allow enumeration of MMC host controllers
even though they have no card(s) inserted.

Patch based on work by Pierre Ossman.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
index 3078623..a74a810 100644
--- a/include/linux/mmc/host.h
+++ b/include/linux/mmc/host.h
@@ -63,6 +63,7 @@
 
 struct mmc_host {
 	struct device		*dev;
+	struct class_device	class_dev;
 	struct mmc_host_ops	*ops;
 	unsigned int		f_min;
 	unsigned int		f_max;