dt-bindings: pinctrl: mcp23s08: Clarify gpiospec flags cell usage

The pinctrl-mcp23s08 driver doesn't provide a specific gpiospec
of_xlate() function, causing the gpiolib-of's of_gpio_simple_xlate()
function to be used instead, which takes the gpiospec's second cell as
the flags specifier according to 'include/dt-bindings/gpio/gpio.h'.

The pinctrl-mcp23s08 bindings document was mentioning that the flags
were unused, which is not accurate because values in that second cell
are indeed used by the gpiolib-of's of_gpio_simple_xlate() for
configuring the gpio (e.g. its polarity).

This way, replace the "flags currently unused" reference in the
dt-bindings document with references to the appropriate files specifying
the possible flag values and gpiospec description.

CC: [e-mail]
Signed-off-by: Ricardo Silva <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/devicetree/bindings/pinctrl/pinctrl-mcp23s08.txt

···
     removed.
 - #gpio-cells : Should be two.
   - first cell is the pin number
-  - second cell is used to specify flags. Flags are currently unused.
+  - second cell is used to specify flags as described in
+    'Documentation/devicetree/bindings/gpio/gpio.txt'. Allowed values defined by
+    'include/dt-bindings/gpio/gpio.h' (e.g. GPIO_ACTIVE_LOW).
 - gpio-controller : Marks the device node as a GPIO controller.
 - reg : For an address on its bus. I2C uses this a the I2C address of the chip.
         SPI uses this to specify the chipselect line which the chip is