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 <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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