arch-arm, dev-arm: Autogen PSCI node in DTB

arch-arm, dev-arm: Autogen PSCI node in DTB

This is controlled via the python only _have_psci parameter
This flag will be checked when auto-generarting a PSCI node. A client
(e.g Linux) would then be able to know if it can use the PSCI APIs

Change-Id: I16c4a67bd358eca3dfff6c98ab8a602a31e1c751
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27387
Tested-by: kokoro <noreply+kokoro@google.com>