dev-arm: Start using GITS_CTLR.quiescent bit

dev-arm: Start using GITS_CTLR.quiescent bit

The GITS_CTLR.quiescent bit is used by priviledged sw to check when the
ITS has finished draining its state (all pending translations/table
walks have ended) once it has been disabled (by setting the
GITS_CTLR.enable bit to 0).
This patch is modelling this behaviour by

* Changing the reset state to enable=0, quiescent=1
* Making the GITS_CTLR.quiescent bit RO
* Updating the bit once a new translation/command is being processed
(quiescent=0) and when there are no pending translation/commands
(quiescent=1)

Change-Id: I7cfe94b25d603400364b1cdfc2d2397acf5dfad8
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20257
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>