For devices that cannot be statically initialized, implement a
get_dt_compatible() callback that allows us to ask the device for
the 'compatible' value.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20200121152935.649898-3-stefanb@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>