bindings: update imx and mxs #gpio-cells

The of_get_named_gpio_flags can retrieve the second cell of
gpio-specifier as the "flags".  The imx and mxs gpio driver do not
have their own .xlate callback, which means of_gpio_simple_xlate is
used and it's a 1:1 mapping between gpio controller flags and Linux
"enum of_gpio_flags".

Update imx and mxs bindings document to make it clear.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

Documentation/devicetree/bindings/gpio/fsl-imx-gpio.txt

@@ -8,8 +8,9 @@ Required properties:
   by low 16 pins and the second one is for high 16 pins.
 - gpio-controller : Marks the device node as a gpio controller.
 - #gpio-cells : Should be two.  The first cell is the pin number and
-  the second cell is used to specify optional parameters (currently
-  unused).
+  the second cell is used to specify the gpio polarity:
+      0 = active high
+      1 = active low
 
 Example:
 

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

@@ -13,8 +13,9 @@ Required properties for GPIO node:
 - interrupts : Should be the port interrupt shared by all 32 pins.
 - gpio-controller : Marks the device node as a gpio controller.
 - #gpio-cells : Should be two.  The first cell is the pin number and
-  the second cell is used to specify optional parameters (currently
-  unused).
+  the second cell is used to specify the gpio polarity:
+      0 = active high
+      1 = active low
 - interrupt-controller: Marks the device node as an interrupt controller.
 - #interrupt-cells : Should be 2.  The first cell is the GPIO number.
   The second cell bits[3:0] is used to specify trigger type and level flags: