arm64 acpi: Add support for IORT table

Add new file arm64/acpica/acpi_iort.c to support the "IO Remapping
Table" (IORT). The table is specified in ARM document "ARM DEN 0049D"
titled "IO Remapping Table Platform Design Document". The IORT table
has information on the associations between PCI root complexes, SMMU
blocks and GIC ITS blocks in the system.

The changes are to parse and save the information in the IORT table.
The API to use this information is added to sys/dev/acpica/acpivar.h.

The acpi_iort.c also has code to check the GIC ITS nodes seen in the
IORT table with corresponding entries in MADT table (for validity)
and with entries in SRAT table (for proximity information).

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D18002