nvic: Support banked exceptions in acknowledge and complete

nvic: Support banked exceptions in acknowledge and complete

Update armv7m_nvic_acknowledge_irq() and armv7m_nvic_complete_irq()
to handle banked exceptions:
 * acknowledge needs to use the correct vector, which may be
   in sec_vectors[]
 * acknowledge needs to return to its caller whether the
   exception should be taken to secure or non-secure state
 * complete needs its caller to tell it whether the exception
   being completed is a secure one or not

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-20-git-send-email-peter.maydell@linaro.org