LinuxKPI: dma-mapping: add dmam_alloc_coherent()

Add the devres version dmam_alloc_coherent() of dma_alloc_coherent()
along with the ancillary free function.

Sponsored by: The FreeBSD Foundation
MFC after: 7 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D36661