On exit, any empty LR will be signaled in GICH_ELRSR*. Which
means that we do not have to save it, and we can just clear
its state in the in-memory copy.

Take this opportunity to move the LR saving code into its
own function.
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>