Allocate PCIe memory regions with rman_manage_region()
Port parts of busdma necessary for pci compilation on arm64.

Porting based on file sys/arm/arm/busdma_machdep.c (SVN rev.273377).
Implement remaining pcib exported functions.

	- Allocate resources during PCIe bus enumeration
	- WAs for determining bus and domain number of a device
	- Remove redundant rman_manage_region calls
	- Minor fixes
	- MSI-X interrupt handling incomplete