pwm: mediatek: Add support for MT7981

The PWM unit on MT7981 uses different register offsets than previous
MediaTek PWM units. Add support for these new offsets and add support
for PWM on MT7981 which has 3 PWM channels, one of them is typically
used for a temperature controlled fan.
While at it, also reorder pwm_mediatek_of_data entries to restore
alphabetic order.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>