arm64: KVM: document kernel object mappings in HYP

HYP mode has access to some of the kernel pages. Document the
memory mapping and the offset between kernel VA and HYP VA.
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Documentation/arm64/memory.txt

@@ -73,3 +73,10 @@ Translation table lookup with 64KB pages:
  |                 |    +--------------------------> [41:29] L2 index (only 38:29 used)
  |                 +-------------------------------> [47:42] L1 index (not used)
  +-------------------------------------------------> [63] TTBR0/1
+
+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):
+
+Start			End			Size		Use
+-----------------------------------------------------------------------
+0000004000000000	0000007fffffffff	 256GB		kernel objects mapped in HYP