dt-bindings: arm: rockchip: Add Firefly Station M2

The Station M2 is a compact single board computer based on the rk3566
SoC. It outputs on uart2 for debug and console purposes.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220429115252.2360496-4-pgwipeout@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>