Create root DMA tag and fix MBUS windows on DMA coherent platforms

Armada 38x SoCs, in order to work properly in IO-coherent mode,
requires an update of the MBUS windows attributesd.

This patch also configures nexus coherent dma tag, because all
busses and children devices have to inherit this setting in runtime.
The latter has to be executed as a sysinit (SI_SUB_DRIVERS type),
so that bus_dma_tag_create() can be executed properly.

Submitted by: Michal Mazur <mkm@semihalf.com>

	      Marcin Wojtas <mw@semihalf.com>

Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: ian
Differential revision: https://reviews.freebsd.org/D11203