usb: udc-core: add is_selfpowered sys entry

The user can read it through sys entry.
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/udc/udc-core.c

@@ -564,6 +564,7 @@ static USB_UDC_ATTR(is_a_peripheral);
 static USB_UDC_ATTR(b_hnp_enable);
 static USB_UDC_ATTR(a_hnp_support);
 static USB_UDC_ATTR(a_alt_hnp_support);
+static USB_UDC_ATTR(is_selfpowered);
 
 static struct attribute *usb_udc_attrs[] = {
 	&dev_attr_srp.attr,
@@ -577,6 +578,7 @@ static struct attribute *usb_udc_attrs[] = {
 	&dev_attr_b_hnp_enable.attr,
 	&dev_attr_a_hnp_support.attr,
 	&dev_attr_a_alt_hnp_support.attr,
+	&dev_attr_is_selfpowered.attr,
 	NULL,
 };