dt-bindings: interrupt-controller: Add Realtek RTL838x/RTL839x support

Document the binding for the Realtek RTL838x/RTL839x interrupt controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bert Vermeulen <bert@biot.com>
[maz: Add a commit message, as the author couldn't be bothered...]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210122204224.509124-2-bert@biot.com