Improve ports handling in e6000sw driver

[bsz_semihalf.com]

Summary

Changes:

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

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

Submitted by: Bartosz Szczepanek <[e-mail]>

		Konrad Adamczyk <[e-mail]>

Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision:
Reviewed by: