net: mtk_eth_soc: convert code structure to suit split PCS support

Provide a mtk_pcs structure which encapsulates everything that the PCS
functions need (the regmap and ana_rgc3 offset), and use this in the
PCS functions. Provide shim functions to convert from the existing
"mtk_sgmii_*" interface to the converted PCS functions.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>