target-arm: Make mode switches from Hyp via CPS and MRS illegal

Mode switches from Hyp to any other mode via the CPS and MRS
instructions are illegal mode switches (though obviously switching
via exception return is valid).  Add this check to bad_mode_switch().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1455556977-3644-11-git-send-email-peter.maydell@linaro.org
