pwm: mediatek: Add MT2712/MT7622 support

Add support for MT2712 and MT7622. Due to register offset address of
pwm7 for MT2712 is not fixed 0x40, add mtk_pwm_reg_offset array for PWM
register offset.

Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Zhi Mao <zhi.mao@mediatek.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>