target/arm: Add decodetree entry for DSB nXS variant

target/arm: Add decodetree entry for DSB nXS variant

The DSB nXS variant is always both a reads and writes request type.
Ignore the domain field like we do in plain DSB and perform a full
system barrier operation.

The DSB nXS variant is part of FEAT_XS made mandatory from Armv8.7.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241211144440.2700268-5-peter.maydell@linaro.org
[PMM: added missing "UNDEF unless feature present" check]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>