nvic: Make SHCSR banked for v8M

Handle banking of SHCSR: some register bits are banked between
Secure and Non-Secure, and some are only accessible to Secure.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-19-git-send-email-peter.maydell@linaro.org