dt-bindings: arm: rockchip: Add Khadas Edge2 board

Edge2 is an ultraslim, credit-card sized ARM PC designed by Khadas.
It has quite a few rich peripherals.

Specification:
- Rockchip RK3588s
- 8/16GB LPDDR4
- 32/64GB eMMC 5.1
- AP6275P WiFi6 LAN
- HDMI2.1 Type-A
- MIPI-CSI x3
- MIPI-DSI x2
- usb3.1; usb2.0
- RTC clock
- PWM fan
- SPI Flash
- Pads expansion board (UART, USB)

Signed-off-by: Yixun Lan <dlan@gentoo.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230315033441.32719-2-dlan@gentoo.org
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -533,6 +533,11 @@ properties:
               - khadas,edge-v
           - const: rockchip,rk3399
 
+      - description: Khadas Edge2 series boards
+        items:
+          - const: khadas,edge2
+          - const: rockchip,rk3588s
+
       - description: Kobol Helios64
         items:
           - const: kobol,helios64