dt-bindings: pwm: mediatek: Remove gratuitous compatible string for MT7629

The MT7629 is, in fact, not compatible with the MT7622 because the
former has a single PWM channel while the former has 6. Remove the
gratuitous compatible string for MT7629.

Reported-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>