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.