dt-bindings: Add doc for Pine64 Pinebook Pro

Add a compatible for Pine64 Pinebook Pro

Signed-off-by: Emmanuel Vadot <manu@freebsd.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200304213023.689983-2-t.schramm@manjaro.org


Signed-off-by: Heiko Stuebner <heiko@sntech.de>