target/arm: Suppress Coverity warning for PRF

These instructions must perform the sve_access_check, but
since they are implemented as NOPs there is no generated
code to elide when the access check fails.

Fixes: Coverity issues 1393780 & 1393779.
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>