net: dsa: rzn1_a5psw: provide own phylink MAC operations

Convert rzn1_a5psw to provide its own phylink MAC operations, thus
avoiding the shim layer in DSA's port.c. We need to provide a stub for
the mac_config() method which is mandatory.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1rwfuJ-00753D-6d@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>