Add usb_fs driver so that userspace can directly transfer USB packets
through the EP node. ADB, Fastboot, MTP will use usb_fs, these class
driver only need to provide the descriptors and register the usb_fs device.
Signed-off-by: zhangyuan21 <zhangyuan21@xiaomi.com>
Signed-off-by: zhangyuan21 <zhangyuan21@xiaomi.com>
