phy: twl4030-usb: add ABI documentation

This driver device one local attribute: vbus.
Describe that in Documentation/ABI/testing/sysfs-platform/twl4030-usb.
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/ABI/testing/sysfs-platform-twl4030-usb

+What: /sys/bus/platform/devices/*twl4030-usb/vbus
+Description:
+	Read-only status reporting if VBUS (approx 5V)
+	is being supplied by the USB bus.
+
+	Possible values: "on", "off".
+
+	Changes are notified via select/poll.