arm64: Update the KVM memory map documentation

Update the documentation to reflect the new tricks we play on the
EL2 mappings...
Acked-by: NChristoffer Dall <christoffer.dall@linaro.org>
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>

Documentation/arm64/memory.txt

@@ -86,9 +86,11 @@ Translation table lookup with 64KB pages:
  +-------------------------------------------------> [63] TTBR0/1
 
 
-When using KVM without the Virtualization Host Extensions, the hypervisor
-maps kernel pages in EL2 at a fixed offset from the kernel VA. See the
-kern_hyp_va macro for more details.
+When using KVM without the Virtualization Host Extensions, the
+hypervisor maps kernel pages in EL2 at a fixed (and potentially
+random) offset from the linear mapping. See the kern_hyp_va macro and
+kvm_update_va_mask function for more details. MMIO devices such as
+GICv2 gets mapped next to the HYP idmap page.
 
 When using KVM with the Virtualization Host Extensions, no additional
 mappings are created, since the host kernel runs directly in EL2.