Enable setting the dma tag at the nexus level

Allow to set the dma tag for nexus in the platform init code,
so that all busses and devices would be able to inherit it.
This change is useful e.g. for setting coherent dma tag for
the platforms with hardware IO cache coherency.

Submitted by: ian

	      Michal Mazur <mkm@semihalf.com>

Reviewed by: ian
Differential revision: https://reviews.freebsd.org/D11202