Break out code to save/restore AArch64 SP into functions.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Greg Bellows <greg.bellows@linaro.org>
Message-id: 1402994746-8328-2-git-send-email-edgar.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>