arm64: Update the KVM memory map documentation

Update the documentation to reflect the new tricks we play on the
EL2 mappings...

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