nvic: Make ICSR.RETTOBASE handle banked exceptions

Update the code in nvic_rettobase() so that it checks the
sec_vectors[] array as well as the vectors[] array if needed.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-7-git-send-email-peter.maydell@linaro.org
