arm64: dts: mt7986: add PWM

This adds pwm node to mt7986.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Link: https://lore.kernel.org/r/20230421132047.42166-5-linux@fw-web.de


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