[arswitch] begin tidying up the learning and ATU management, introduce ATU APIs.

• Refactor the initial learning configuration (port learning, address expiry, handling address moving between ports, etc, etc) into a separate HAL routine
• and ensure that it's consistent between switch chips - the AR8216,8316,724x,9331 SoCs all share the same switch code.
• .. the AR8327 needs doing - the defaults seem OK for now
• .. the AR9340 is different but it's also programmed now.

• Add support for flushing a single port worth of ATU entries
• Add support for fetching the ATU table from AR8216 and derived chips

Tested:

• AR9344, Carambola 2

TODO:

• Further testing on other chips
• Add AR9340 support
• Add AR8327 support