greybus: interface: document sysfs files

Yes, an interface has a device id sysfs file, so we need to document it.
Reviewed-by: NAlex Elder <elder@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/Documentation/sysfs-bus-greybus

@@ -53,3 +53,10 @@ KernelVersion:	3.XX
 Contact:	Greg Kroah-Hartman <greg@kroah.com>
 Description:
 		The protocol id of a Greybus connection.
+
+What:		/sys/bus/greybus/device/.../device_id
+Date:		December 2014
+KernelVersion:	3.XX
+Contact:	Greg Kroah-Hartman <greg@kroah.com>
+Description:
+		The device id of a Greybus interface.