Add instructions of SC opcode format.
Add helper for begin interrupt service routine.

Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Message-id: 1409572800-4116-14-git-send-email-kbastian@mail.uni-paderborn.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>