target/arm: Fix PAuth keys access checks for disabled SEL2

As per the description of the HCR_EL2.APK field in the ARMv8 ARM,
Pointer Authentication keys accesses should only be trapped to Secure
EL2 if it is enabled.

Signed-off-by: Florian Lugou <florian.lugou@provenrun.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220517145242.1215271-1-florian.lugou@provenrun.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>