Enable etherswitchcfg and e6000sw driver in arm64 build

After latest binding update, this patch enables usage of
the switch on Armada 3720 EspressoBin, so compile it
by default with arm64 GENERIC.

A patch was extracted from https://reviews.freebsd.org/D19036

Submitted by: Bert JW Regeer <xistence@0x58.com>
Reviewed by: manu