Introduce IOMMU support for arm64 platform.

This adds an arm64 iommu interface and a driver for Arm System Memory
Management Unit version 3.2 (ARM SMMU v3.2) specified in ARM IHI 0070C
document.

Hardware overview is provided in the header of smmu.c file.

The support is disabled by default. To enable add 'options IOMMU' to your
kernel configuration file.

The support was developed on Arm Neoverse N1 System Development Platform
(ARM N1SDP), kindly provided by ARM Ltd.

Currently, PCI-based devices and ACPI platforms are supported only.
The support was tested on IOMMU-enabled Marvell SATA controller,
Realtek Ethernet controller and a TI xHCI USB controller with a low to
medium load only.

Many thanks to Konstantin Belousov for help forming the generic IOMMU
framework that is vital for this project; to Andrew Turner for adding
IOMMU support to MSI interrupt code; to Mark Johnston for help with SMMU
page management; to John Baldwin for explaining various IOMMU bits.

Reviewed by: mmel
Relnotes: yes
Sponsored by: DARPA / AFRL
Sponsored by: Innovate UK (Digital Security by Design programme)
Differential Revision: https://reviews.freebsd.org/D24618