greybus: Documentation: Add sysfs information about bridged-phy device

Update the documentation (sysfs-bus-greybus) for recent conversion of
bridged-phy drivers to real drivers.
Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

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

@@ -162,6 +162,14 @@ Contact:	Greg Kroah-Hartman <greg@kroah.com>
 Description:
 		The interface-unique id of the bundle B.
 
+What:		/sys/bus/greybus/devices/N-M.I.B/gpbX
+Date:		April 2016
+KernelVersion:	4.XX
+Contact:	Greg Kroah-Hartman <greg@kroah.com>
+Description:
+		The General Purpose Bridged PHY device of the bundle B,
+		where X is a dynamically assigned 0-based id.
+
 What:		/sys/bus/greybus/devices/N-M.I.B/state
 Date:		October 2015
 KernelVersion:	4.XX