net: phy: realtek: add get_rate_matching() for rtl822xb PHYs

Uses vendor register to determine if SerDes is setup in rate-matching mode.

Rate-matching only supported when SerDes is set to 2500base-x.

Signed-off-by: Eric Woudstra <ericwouds@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>