dt-bindings: arm: Add Topwise A721

Add the bindings for Topwise A721 tablet

Signed-off-by: Pascal Roeleven <dev@pascalroeleven.nl>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210224105240.47754-2-dev@pascalroeleven.nl