Improve ports handling in e6000sw driver

• recognize ports and vlangroups based on DTS file
• support multi-chip addresing mode (required in upcoming Armada-388-Clearfog support)
• refactor attachment function

Each port in 'dsa' node should have 'vlangroup' property. Otherwise,
e6000sw will fail to attach.

Submitted by: Bartosz Szczepanek <bsz@semihalf.com>

		Konrad Adamczyk <ka@semihalf.com>

Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D7328