net: dsa: mt7530: use external PCS driver

Implement regmap access wrappers, for now only to be used by the
pcs-mtk-lynxi driver.
Make use of this external PCS driver and drop the now reduntant
implementation in mt7530.c.
As a nice side effect the SGMII registers can now also more easily be
inspected for debugging via /sys/kernel/debug/regmap.

Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>