AMD IOMMU driver

This driver is functionally equivalent to the in-tree Intel DMAR code.
It handles busdma and interrupt remapping from the host OS. There is no
integration with bhyve, and the stub iommu drivers in bhyve code cannot
coexist with this driver (planned).

The biggest architectural problem with the code is that the AMD IOMMU
units are enumerated as PCIe-attached security devices, which is much
later after HPET and IOAPIC drivers attached and actived interrupts.
Because of this, HPET FSB interrupts and IOAPIC interrupts are always
identity-mapped.

The code is of late alpha quality. By default the driver is disabled.
To enable for testing, set in loader.conf:
hw.amdiommu.enable=1
hw.iommu.dma=1 <- to enable iommu busdma
hw.iommu.ir=1 <- to enable interrupt remapping

Discussed with: emaste
Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D47256