net: phy: marvell10g: add MDI swap reporting

Add reporting of the MDI swap to the Marvell 10G PHY driver by providing
a generic implementation for the standard 10GBASE-T pair swap register
and polarity register.  We also support reading the MDI swap status for
1G and below from a PCS register.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>