backlight: hx8357: Make use of device properties

Convert the module to be property provider agnostic and allow
it to be used on non-OF platforms.

Include mod_devicetable.h explicitly to replace the dropped of.h
which included mod_devicetable.h indirectly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240201144951.294215-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Lee Jones <lee@kernel.org>