[][kernel][common][pwm][Remove unused cell of pwm in dts]

[Description]
Remove unused cell of pwm in dts, b/c 7986/7981/7988 PWM only
supports 2 cells channel and period: "#pwm-cells = <2>;".

[Release-log]
N/A

Change-Id: I46ca2503a3795e8b16b574bc150474fd094470f9
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8477301
32 files changed