regulator: dt-bindings: Add Awinic AW37503

Add aw37503 regulator device-tree binding documentation.

Signed-off-by: Alec Li <like@awinic.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230821035355.1269976-3-like@awinic.com


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