MFC r347337:
o Implement a bounce buffer based on device reserved memory.
Grab device reserved physical memory regions from FDT using standard "memory-region" property and use vmem(9) to allocate buffers from it. The same vmem could be used by DMA engine drivers to allocate memory for DMA descriptors. This is required for platforms that provide uncached memory region reserved exclusively for DMA operations.
o Change sleepable sx(9) lock type to non-sleepable mutex(9) since
network drivers usually hold mutex during DMA operations. So we don't take sleepable lock after non-sleepable.
Tested on U.S. Government Furnished Equipment (GFE) 64-bit RISC-V cores.
Sponsored by: DARPA, AFRL