doc/bindings: Update GPIO devicetree binding documentation for LS2080A

Update the FSL, GPIO binding documentation to add support
for GPIO controller found on Freescale's LS2080A platform.
Signed-off-by: Liu Gang <Gang.Liu@freescale.com>
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/gpio/gpio-mpc8xxx.txt

-* Freescale MPC512x/MPC8xxx GPIO controller
+* Freescale MPC512x/MPC8xxx/Layerscape GPIO controller
 
 Required properties:
 - compatible : Should be "fsl,<soc>-gpio"
   The following <soc>s are known to be supported:
-    mpc5121, mpc5125, mpc8349, mpc8572, mpc8610, pq3, qoriq
+    mpc5121, mpc5125, mpc8349, mpc8572, mpc8610, pq3, qoriq.
 - reg : Address and length of the register set for the device
 - interrupts : Should be the port interrupt shared by all 32 pins.
 - #gpio-cells : Should be two.  The first cell is the pin number and