dt-bindings: sunxi: Add Allwinner A80 PRCM Binding

Even though we've used the A80 PRCM driver for some time and a number of
boards using it already, we never had a binding for it. Let's add it
based on what the driver expects and the boards are providing.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20210901091852.479202-36-maxime@cerno.tech