ASoC: dt-bindings: add compatible string for NAU8318

The audio amplifier NAU8318 is almost functionally identical to NAU8315.
Adds compatible string "nuvoton,nau8318" for driver reuse.

Signed-off-by: David Lin <CTLIN0@nuvoton.com>
Link: https://lore.kernel.org/r/20221124055658.53828-2-CTLIN0@nuvoton.com


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