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

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: devicetree@vger.kernel.org
Signed-off-by: Ricardo Silva <rjpdasilva@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>