arm64: dts: rockchip: Add pdm node to rk356x

rk356x contains a PDM microphone controller which is compatible with the
existing rockchip,pdm binding. Add its node.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20220130053803.43660-2-samuel@sholland.org


Signed-off-by: Heiko Stuebner <heiko@sntech.de>