armv7m: Honour CCR.USERSETMPEND

The CCR.USERSETMPEND bit has to be set to permit unprivileged code to
write to the Software Triggered Interrupt register; honour this bit
rather than letting any code write to the register.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1485285380-10565-9-git-send-email-peter.maydell@linaro.org
[PMM: Tweak commit message, comment, phrasing of condition]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>