ASoC: dt-bindings: rockchip: Add binding for rk3568 pdm

This patch documents for rk3568 pdm.
Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1630675410-3354-4-git-send-email-sugar.zhang@rock-chips.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -6,6 +6,7 @@ Required properties:
   - "rockchip,px30-pdm"
   - "rockchip,rk1808-pdm"
   - "rockchip,rk3308-pdm"
+  - "rockchip,rk3568-pdm"
   - "rockchip,rv1126-pdm"
 - reg: physical base address of the controller and length of memory mapped
   region.