system-arm: bigLITTLE with VExpress_GEM5_V2 in dtb

system-arm: bigLITTLE with VExpress_GEM5_V2 in dtb

This patch adds targets in the device tree Makefile for using
bigLITTLE DTS with VExpress_GEM5_V2 platform.

Change-Id: I7a424a36c78a24b96224526aa112ac5d060f790b
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24083
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>