net: dsa: mt7530: partially convert to phylink_pcs

Partially convert the mt7530 driver to use phylink's PCS support. This
is a partial implementation as we don't move anything into the
pcs_config method yet - this driver supports SGMII or 1000BASE-X
without in-band.

Tested-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>