dm: mmc: Use bootdev_setup_sibling_blk()

At present MMC uses the bootdev_setup_for_dev() function to set up the
bootdev. This is because MMC only has one block-device child, so does not
need to worry about naming of the bootdev.

However this inconsistency with other bootdevs that use block devices is a
bit annoying. The only real reason for it is to have a name like
'mmc0.bootdev' instead of 'mmc0.blk.bootdev'.

Update bootdev_setup_sibling_blk() to drop '.blk' from the name where it
appears, thus removing the only reason to use the bootdev_setup_for_dev().
Switch MMC over to the subling function.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/bootdev.h

@@ -204,7 +204,10 @@ int bootdev_setup_iter_order(struct bootflow_iter *iter, struct udevice **devp);
 
 #if CONFIG_IS_ENABLED(BOOTSTD)
 /**
- * bootdev_setup_for_dev() - Bind a new bootdev device
+ * bootdev_setup_for_dev() - Bind a new bootdev device (deprecated)
+ *
+ * Please use bootdev_setup_sibling_blk() instead since it supports multiple
+ * (child) block devices for each media device.
  *
  * Creates a bootdev device as a child of @parent. This should be called from
  * the driver's bind() method or its uclass' post_bind() method.