net: dsa: b53: populate supported_interfaces and mac_capabilities

Populate the supported interfaces and MAC capabilities for the Broadcom
B53 DSA switches in preparation to using these for the generic
validation functionality.

The interface modes are derived from:
- b53_serdes_phylink_validate()
- SRAB mux configuration

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