dt-bindings: arm64: rockchip: Add Pine64 PineTab2

Add devicvetree binding documentation for Pine64 PineTab2
which uses the Rockchip RK3566 SoC.

Signed-off-by: Manuel Traut <manut@mecka.net>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240127-pinetab2-v4-3-37aab1c39194@mecka.net
[moved Pinetab below Pinephone to keep alphabetical sorting]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>