net: dsa: xrs700x: provide own phylink MAC operations

Convert xrs700x to provide its own phylink MAC operations, thus
avoiding the shim layer in DSA's port.c. We need to provide stubs for
the mac_link_down() and mac_config() methods which are mandatory.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1rwfu8-007531-TG@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>