gpio: documenatation: fix GPIO_LOOKUP{,_IDX} documentation

The 'dev_id' parameter of the GPIO_LOOKUP{,_IDX} macros were removed by
commit ad824783 ("gpio: better lookup method for platform GPIOs").

Update the documentation to reflect that.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>