backlight: ktz8866: Add support for Kinetic KTZ8866 backlight

backlight: ktz8866: Add support for Kinetic KTZ8866 backlight

Add support for Kinetic KTZ8866 backlight, which is used in
Xiaomi tablet, Mi Pad 5 series. This driver lightly based on
downstream implementation [1].
[1] https://github.com/MiCode/Xiaomi_Kernel_OpenSource/blob/elish-r-oss/drivers/video/backlight/ktz8866.c

Signed-off-by: Jianhua Lu <lujianhua000@gmail.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230120155018.15376-2-lujianhua000@gmail.com