dpaa2-mac: split up initializing the MAC object from connecting to it

Split up the initialization phase of the dpmac object from actually
configuring the phylink instance, connecting to it and configuring the
MAC. This is done so that even though the dpni object is connected to a
dpmac which has link management handled by the firmware we are still
able to export the MAC counters.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>