dpaa2-switch: refactor the egress flooding domain setup

Extract the code that determines the list of egress flood interfaces for
a specific flood type into a new function -
dpaa2_switch_fdb_get_flood_cfg().

This will help us to not duplicate code when the broadcast and
unknown ucast/mcast flooding domains will be individually configurable.

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