net: dsa: xrs700x: convert to phylink_generic_validate()

Populate the supported interfaces and MAC capabilities for the xrs700x
family of DSA switches and remove the old validate implementation to
allow DSA to use phylink_generic_validate() for this switch driver.

According to commit ee00b24f ("net: dsa: add Arrow SpeedChips
XRS700x driver") the switch supports one RMII port and up to three
RGMII ports. This commit assumes that port 0 is the RMII port and the
remainder are RGMII.

This commit also results in the Autoneg bit being set in the ethtool
link modes, which wasn't in the original; if this switch supports
RGMII to a 10/100/1G PHY, then surely we want to allow Autoneg on the
PHY.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>