target/arm: Move regime_is_secure() to target/arm/internals.h

Move the regime_is_secure() utility function to internals.h;
we are going to want to call it from translate.c.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1503414539-28762-20-git-send-email-peter.maydell@linaro.org