gpio: generic: add request function pointer

gpiolib will require all gpio drivers to expicitly set the request
function pointer in the future. To encourage gpio driver developers
to adhere to this standard gpio-generic.c now sets this function
pointer.

Signed-off-by: Anthony Fee <anthony.fee@emutex.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>