ARM: KVM: allow HYP mappings to be at an offset from kernel mappings

arm64 cannot represent the kernel VAs in HYP mode, because of the lack
of TTBR1 at EL2. A way to cope with this situation is to have HYP VAs
to be an offset from the kernel VAs.

Introduce macros to convert a kernel VA to a HYP VA, make the HYP
mapping functions use these conversion macros. Also change the
documentation to reflect the existence of the offset.

On ARM, where we can have an identity mapping between kernel and HYP,
the macros are without any effect.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>