fastmodel: Implement GIC DTB auto-generation.

fastmodel: Implement GIC DTB auto-generation.

Implement generateDeviceTree for FastModelGIC so the interrupt
controller is automatically added to the DTB. This is sufficient to
allow a VExpressFastmodel system model to boot Linux without an
explicit DTB.

Change-Id: I69d86fd8bba1b86768c8a118d2de079a56179854
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31078
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>