e6000sw: add support for 88E6190X

This adds the minimum support required to probe/attach the 88E6190X.

I've tested this against an AT&T ATT-150 OCP device (Silicom i3000)
with local changes to export MDIO via ixge(4).

Hints are required to probe/attach/configure the switch on amd64,
but with the mentioned diffs, it does work.

Thanks to Stas Alekseev <stas@alekseev.us> for the pull request
and Stas / Jason Hensler <omegadraconis@gmail.com> for chasing
down information about the chipset, linux stuff and AT&T OCP
hardware information.

PR: kern/281211
Pull Request: https://github.com/freebsd/freebsd-src/pull/1408
Differential Revision: https://reviews.freebsd.org/D50044
Reviewed by: imp