Document the reserved status
Document the reserved status

We use this on Bare metal Power 8/9 systems to mark devices that
are used by the OPAL firmware rather than being available to the
operating system.

Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

devicetree-org/devicetree-specification