[ath] add WB335 btcoex for initial testing.

This is like the WB222 coexistence (ie, "MCI", a message bus inside the
chip), and it's currently a cut/paste so I can start using it to flesh
out the differences with WB222.

It doesn't completely /do/ bluetooth coexistence, because it turns out
I need to add some contigmalloc'ed buffers to the btcoex path for this
type of hardware. I'm putting this work in the "people would like
to see functioning-ish btcoex before FreeBSD-11" bucket because I see
this as "broken".

Tested:

• QCA9535 (WB335) NIC, BT + 2GHz STA