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.com


Signed-off-by: Mark Brown <broonie@kernel.org>