Introduce support for DMA coherent ARM platforms

• Inherit BUS_DMA_COHERENT flag from parent buses
• Use cacheable memory attributes on dma coherent platform
• Disable cache synchronization on coherent platform

Changes are based on ARMv8 busdma code and commit r299683.

Submitted by: Michal Mazur <mkm@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: ian
Differential revision: https://reviews.freebsd.org/D11201