arm64: dts: mediatek: spherion: Add keyboard backlight

The Spherion board has keyboard backlight controlled by the PWM signal
generated by the ChromeOS EC.

Enable PWM output for ChromeOS EC and add a PWM controlled LED node for
the keyboard backlight.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20220629155956.1138955-13-nfraprado@collabora.com


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