armv7-m: Implement SYSRESETREQ

Implement the SYSRESETREQ bit of the AIRCR register
for armv7-m (ie. cortex-m3) to trigger a GPIO out.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>