net: ethernet: mtk_eth_soc: switch to external PCS driver

Now that we got a PCS driver, use it and remove the now redundant
PCS code and it's header macros from the Ethernet driver.

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>