dm: mmc: Use bootdev_setup_sibling_blk()

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>