dt-bindings: backlight: lp855x: Convert to YAML and modernize

Notable changes:
- ROM child nodes use dashes instead of underscores; the driver
  reads all child nodes regardless of their names, so this doesn't
  break ABI.
- pwm-period argument is deprecated, as it effectively duplicates
  the period value provided in pwms. The driver continues to accept
  the property, so this should not break ABI.

Signed-off-by: Artur Weber <aweber.kernel@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230519180728.2281-2-aweber.kernel@gmail.com