arm/ptw: Honour WXN/UWXN and SIF in short-format descriptors

arm/ptw: Honour WXN/UWXN and SIF in short-format descriptors

Currently the handling of page protection in the short-format
descriptor is open-coded. This means that we forgot to update
it to handle some newer architectural features, including:
 * handling of SCTLR.{UWXN,WXN}
 * handling of SCR.SIF

Make the short-format descriptor code call the same get_S1prot()
that we already use for the LPAE descriptor format. This makes
the code simpler and means it now correctly honours the WXN/UWXN
and SIF bits.

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Message-id: 20241118152537.45277-1-paskripkin@gmail.com
[PMM: fixed a couple of checkpatch nits, tweaked commit message]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>