net: mtk_eth_soc: add pcs_get_state() implementation

Add a pcs_get_state() implementation which uses the advertisements
to compute the resulting link modes, and BMSR contents to determine
negotiation and link status.

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