Split ARM on/off function from PSCI support code.

This will allow to reuse these functions in other code.
Signed-off-by: NJean-Christophe Dubois <jcd@tribudubois.net>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>