* Rockchip PDM controller

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.
- dmas: DMA specifiers for rx dma. See the DMA client binding,
	Documentation/devicetree/bindings/dma/dma.txt
- dma-names: should include "rx".
- clocks: a list of phandle + clock-specifer pairs, one for each entry in clock-names.
- clock-names: should contain following:
   - "pdm_hclk": clock for PDM BUS
   - "pdm_clk" : clock for PDM controller
- resets: a list of phandle + reset-specifer paris, one for each entry in reset-names.
- reset-names: reset names, should include "pdm-m".
- pinctrl-names: Must contain a "default" entry.
- pinctrl-N: One property must exist for each entry in
	     pinctrl-names. See ../pinctrl/pinctrl-bindings.txt
	     for details of the property values.

Example for rk3328 PDM controller:

pdm: pdm@ff040000 {
	compatible = "rockchip,pdm";
	reg = <0x0 0xff040000 0x0 0x1000>;
	clocks = <&clk_pdm>, <&clk_gates28 0>;
	clock-names = "pdm_clk", "pdm_hclk";
	dmas = <&pdma 16>;
	#dma-cells = <1>;
	dma-names = "rx";
	pinctrl-names = "default", "sleep";
	pinctrl-0 = <&pdmm0_clk
		     &pdmm0_sdi0
		     &pdmm0_sdi1
		     &pdmm0_sdi2
		     &pdmm0_sdi3>;
	pinctrl-1 = <&pdmm0_clk_sleep
		     &pdmm0_sdi0_sleep
		     &pdmm0_sdi1_sleep
		     &pdmm0_sdi2_sleep
		     &pdmm0_sdi3_sleep>;
};