arm64: KVM: Add a HYP-specific header file

In order to expose the various EL2 services that are private to
the hypervisor, add a new hyp.h file.

So far, it only contains mundane things such as section annotation
and VA manipulation.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
(cherry picked from commit c76a0a66)
Signed-off-by: Alex Shi <alex.shi@linaro.org>