BCM270X_DT: Add non-removable clone of mmc node

non-removable is a boolean property, and as such can't be unset by an
overlay if it is set in a base DTB. Until now the workaround for this
problem has been for overlays to clone non-removable nodes without
the offending property, but this involves a lot of unnecessary
replication. Instead, add a clone of the mmc node with non-removable
already set to the base DTB, selecting the required version using
the status properties.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>