KVM: arm64: re-factor hyp.S debug register code

This is a pre-cursor to sharing the code with the guest debug support.
This replaces the big macro that fishes data out of a fixed location
with a more general helper macro to restore a set of debug registers. It
uses macro substitution so it can be re-used for debug control and value
registers. It does however rely on the debug registers being 64 bit
aligned (as they happen to be in the hyp ABI).

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>