Allow ARMv8 SCR.SMD updates

Updated scr_write to always allow updates to the SCR.SMD bit on ARMv8
regardless of whether virtualization (EL2) is enabled or not.

Signed-off-by: Greg Bellows <greg.bellows@linaro.org>
Message-id: 1429888797-4378-1-git-send-email-greg.bellows@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>