arm64: dts: mediatek: cherry: Add configuration for display backlight

Configure the hardware PWM for the integrated display's backlight:
all Cherry devices enable the backlight with GPIO82 and manage the
PWM via MediaTek disp-pwm on GPIO97.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20230223145426.193590-5-angelogioacchino.delregno@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>