Add DMIC slew rate controls

Merge series from Seven Lee <wtli@nuvoton.com>:

Determine DMIC slew rate via property setup.

Change:
V3 -> V4:
 - add "maximum: 7" description.

V2 -> V3:
 - Update description of DMIC slew rate and remove
   "selection" key words from property name
 - Corrected variable name of DMIC slew rate from c file

V1 -> V2:
 - Corrected description of DMIC slew rate.

Seven Lee (2):
  ASoC: dt-bindings: nau8821: Add DMIC slew rate.
  ASoC: nau8821: Add slew rate controls.

 .../devicetree/bindings/sound/nuvoton,nau8821.yaml       | 9 +++++++++
 sound/soc/codecs/nau8821.c                               | 7 +++++++
 sound/soc/codecs/nau8821.h                               | 3 +++
 3 files changed, 19 insertions(+)

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2.25.1