ASoC: wm8962: Add optional mclk device tree binding

ASoC: wm8962: Add optional mclk device tree binding

The driver can request an optional clock for mclk.
Update the txt file to reflect this.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201217162740.1452000-1-aford173@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>