arm64: Hyper-V: vPCI: SPI MSI mapping for gic v3 acpi in arm64

Microsoft Azure Hyper-V uses SPI to map MSI in ARM64, instead of
using LPI IDS. To enable that we need to have gic registered with
ACPI_MSI_XREF and gic acpi to map SPI for MSI.

This is the 1st of the three patchs to enable Hyper-V vPCI support
in arm64.

Reviewed by: andrew, emaste, whu
Tested by: Souradeep Chakrabarti <schakrabarti@microsoft.com>
Obtained from: Souradeep Chakrabarti <schakrabarti@microsoft.com>
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D37763