[][kernel][common][pwm][Add pwm-gpio as software pwm]

[Description]
Add pwm-gpio support as software pwm.
It's generic pwm framework driver for a software pwm toggling a GPIO pin
from kernel high-resolution timers.
Reference: https://lore.kernel.org/all/20200902121236.20514-1-vincent
.whitchurch@axis.com

[Release-log]
N/A

Change-Id: I49437cd0e3b027fbec9620ea285fdb6a61bec60f
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8471747
4 files changed