dev/fdt: Add support for non-PCI MSI interrupts

Some non-PCI devices can send interrupts, e.g. the Arm SMMU or GICv5
Interrupt Wire Bridge. Add support for these by implementing pci_get_id
and pci_alloc_msi and the MSI/MSI-X parts of the PCIB interface.

Only the MSI parts of the PCI interface are added as that is all I am
able to test.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D53330