arm64: Document SW reserved PTE/PMD bits in Stage-2 descriptors

Advertise bits [58:55] as reserved for SW in the S2 descriptors.

Reviewed-by: Andrew Scull <ascull@google.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>