net: mtk_eth_soc: add support for in-band 802.3z negotiation

As a result of help from Frank Wunderlich to investigate and test, we
now know how to program this PCS for in-band 802.3z negotiation. Add
support for this by moving the contents of the two functions into the
common mtk_pcs_config() function and adding the register settings for
802.3z negotiation.

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