ARM: KVM: Add a HYP-specific header file

In order to expose the various HYP 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.

Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>