ath79: Fix glinet ar300m usb not working

ath79: Fix glinet ar300m usb not working

glinet forum users reported the problem at
https://forum.gl-inet.com/t/gl-ar300m16-openwrt-22-03-0-rc5-usb-port-power-off-by-default/23199

The current code uses the regulator framework to control the USB power
supply. Although usb0 described in DTS refers to the regulator by
vbus-supply, but there is no code related to regulator implemented
in the USB driver of QCA953X, so the USB of the device cannot work.

Under the regulator framework, adding the regulator-always-on attribute
fixes this problem, but it means that USB power will not be able to be
turned off. Since we need to control the USB power supply in user space,
I didn't find any other better way under the regulator framework of Linux,
so I directly export gpio.

Signed-off-by: Luo Chongjun <luochongjun@gl-inet.com>