system-arm: Use dts include instead of cpp in ARM DTBs

system-arm: Use dts include instead of cpp in ARM DTBs

Change-Id: I342691a42e84dfe53659a7acb3b8db04e52e3002
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22824
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>