dev-arm: Set/Unset dma coherent mode from python

dev-arm: Set/Unset dma coherent mode from python

With this patch it will be possible to automatically enable/disable the
dma-coherent property for the GenericArmPciHost autogenerated DTB.  This
has been done by adding the _dma_coherent param.

Change-Id: I1759fced74e42410462637ca77997a351314a90a
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/16748
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>