dt-bindings: Add doc for ROCK Pi 4 A+ and B+

ROCK Pi 4 got 2 more variants called A+ and B+.
Add the dt-bindings documentation for it.

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210618181256.27992-2-knaerzche@gmail.com


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