hw/arm/smmuv3: Advertise SMMUv3.1-XNX feature

The SMMUv3.1-XNX feature is mandatory for an SMMUv3.1 if S2P is
supported, so we should theoretically have implemented it as part of
the recent S2P work.  Fortunately, for us the implementation is a
no-op.

This feature is about interpretation of the stage 2 page table
descriptor XN bits, which control execute permissions.

For QEMU, the permission bits passed to an IOMMU (via MemTxAttrs and
IOMMUAccessFlags) only indicate read and write; we do not distinguish
data reads from instruction reads outside the CPU proper.  In the
SMMU architecture's terms, our interconnect between the client device
and the SMMU doesn't have the ability to convey the INST attribute,
and we therefore use the default value of "data" for this attribute.

We also do not support the bits in the Stream Table Entry that can
override the on-the-bus transaction attribute permissions (we do not
set SMMU_IDR1.ATTR_PERMS_OVR=1).

These two things together mean that for our implementation, it never
has to deal with transactions with the INST attribute, and so it can
correctly ignore the XN bits entirely.  So we already implement
FEAT_XNX's "XN field is now 2 bits, not 1" behaviour to the extent
that we need to.

Advertise the presence of the feature in SMMU_IDR3.XNX.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Mostafa Saleh <smostafa@google.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-id: 20230914145705.1648377-4-peter.maydell@linaro.org