net: dsa: microchip: lan937x: add MTU and fast_age support

This patch add the support for port_max_mtu, port_change_mtu and
port_fast_age dsa functionality.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>