dpaa2-mac: move setting up supported_interfaces into a function

The logic to setup the supported interfaces will get annotated based on
what the configuration of the SerDes PLLs supports. Move the current
setup into a separate function just to try to keep it clean.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>