hw/arm/smmu: Use enum for SMMU stage

hw/arm/smmu: Use enum for SMMU stage

Currently, translation stage is represented as an int, where 1 is stage-1 and
2 is stage-2, when nested is added, 3 would be confusing to represent nesting,
so we use an enum instead.

While keeping the same values, this is useful for:
 - Doing tricks with bit masks, where BIT(0) is stage-1 and BIT(1) is
   stage-2 and both is nested.
 - Tracing, as stage is printed as int.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Mostafa Saleh <smostafa@google.com>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Message-id: 20240715084519.1189624-5-smostafa@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>