arm64: fix documentation on kernel pages mappings to HYP VA

The Documentation/arm64/memory.txt says:
When using KVM, the hypervisor maps kernel pages in EL2, at a fixed
offset from the kernel VA (top 24bits of the kernel VA set to zero):

In fact, kernel addresses are transleted to HYP with kern_hyp_va macro,
which has more options, and none of them assumes clearing of top 24bits
of the kernel VA.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
[will: removed gory details]
Signed-off-by: Will Deacon <will.deacon@arm.com>