Add support for the GICv2M extension to the GICv2 interrupt controller.
This is (oddly) specified in the ARM Server Base System Architecture. It
extends the GICv2 to support MSI and MSI-X interrupts, however only the
latter are currently supported.

Only the FDT attachment is currently supported, however the attachment
and core driver are split to help adding ACPI support in the future.

Obtained from: ABT Systems Ltd
Relnotes: yes
Sponsored by: SoftIron Inc