dt-bindings: sound: add compatible for rk1808

This patch adds bindings for rk1808 soc.
Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/rockchip,pdm.txt

@@ -4,6 +4,7 @@ Required properties:
 
 - compatible: "rockchip,pdm"
   - "rockchip,px30-pdm"
+  - "rockchip,rk1808-pdm"
   - "rockchip,rk3308-pdm"
 - reg: physical base address of the controller and length of memory mapped
   region.