Improve detection of addressing mode in e6000sw

Some devices cannot rely on the switch MDIO address passed in the DTB
for specifying single/multi-chip addressing mode. Introduce new property
"single-chip-addressing" which added to DTS will force single-chip mode.

Submitted by: Michal Mazur <mkm@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D14800