Convert selectors of GENERIC_GPIO to GPIOLIB

GENERIC_GPIO is now equivalent to GPIOLIB and features that depended on
GENERIC_GPIO can now depend on GPIOLIB to allow removal of this option.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Remove GENERIC_GPIO config option

GENERIC_GPIO has been made equivalent to GPIOLIB in architecture code
and all driver code has been switch to depend on GPIOLIB. It is thus
safe to have GENERIC_GPIO removed.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Rename CONFIG_GPIOLIB to CONFIG_GPIO

The removal of CONFIG_GENERIC_GPIO turned gpiolib into the only way to
implement the generic GPIO API, effectively making it the "GPIO
framework".

This patch tries to reflect this reality by changing occurences of
GPIOLIB into the simpler GPIO in various places (notable configuration
options) and updating the documentation.

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