kernel: net: phy: realtek: replace in-band AN hack

kernel: net: phy: realtek: replace in-band AN hack

Replace downstream hack for RealTek PHYs with a more clean solution
which could make it upstream.

As SGMII in-band AN is broken on some platforms, or simply expected to
be disabled by default in phy/sgmii mode (ie. on-board PHYs with MDIO
for out-of-band configuration and status), a hack for the RealTek PHY
driver was introduced to unconditionally disable SGMII in-band
autonegotiation.

Meanwhile the kernel has gained a proper interface for PHY and PCS to
report in-band AN capabilities and enable/disable in-band, matching
PHY and PCS capabilities.

Thanks to Bevan Weiss' knowledge about how RealTek PHY SerDes registers
are being handled in RealTek's SDK this can now be greatly improved:
 - report in-band capabilties
 - let phylink set in-band matching PCS and PHY capabilities
 - properly abstracted indirect access of SerDes registers

Signed-off-by: Daniel Golle <daniel@makrotopia.org>