pwm: mediatek: Add support for MT7986

Add support for PWM on MT7986 which has 2 PWM channels, one of them is
typically used for a temperature controlled fan.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Sam Shih <sam.shih@mediatek.com>
Link: https://lore.kernel.org/r/Y1K5ym1EL8kwzQEt@makrotopia.org


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>