target/arm/helper: Check SCR_EL3.{NSE, NS} encoding for AT instructions

target/arm/helper: Check SCR_EL3.{NSE, NS} encoding for AT instructions

The AT instruction is UNDEFINED if the {NSE,NS} configuration is
invalid. Add a function to check this on all AT instructions that apply
to an EL lower than 3.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Message-id: 20230809123706.1842548-6-jean-philippe@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>