[arswitch] Enable ATU dump support for the AR9340.

This indeed uses the same registers as the AR8216 and later chips.

There seems to be an issue with ARP requests being sent out from the CPU
through this switch here, so figuring that out is next. Learning works fine on
the AR8327 ethernet switch on the /other/ gigabit ethernet port, so I don't
think it's the network stack or ethernet driver.

Tested:

• DB120 - AR9340 SOC + ethernet switch (and other bits.)