spapr/drc: Drop spapr_drc_attach() fdt argument

All DRC subtypes have been converted to generate the FDT fragment at
configure connector time instead of attach time. The fdt and fdt_offset
arguments of spapr_drc_attach() aren't needed anymore. Drop them and
make the implementation of the dt_populate() method mandatory.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <155059667853.1466090.16527852453054217565.stgit@bahia.lab.toulouse-stg.fr.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>