Coherent bus_dma for ARMv7

[Marcin Wojtas]

Hi,

In current FreeBSD-HEAD ARMv7 platforms, which support hardware IO
cache coherency cannot make use of this feature. In our project we
implemented coherent variant bus_dma, which is basing on x86 and arm64
approach. Using of above solution is not obligatory and depends on
setting newly added option for that purpose.

Needless to say, our platform (Marvell Armada 38x) IO performance
boosted after switching to it. Do you wish to enable such option in
HEAD?

Regards,
Marcin