gpiolib: make GPIO_DEVRES depend on GPIOLIB

Current Kconfig allows GPIO_DEVRES to be selected and compiled without
GPIOLIB. This does not make sense anymore since GPIOLIB has become the
exclusive way to deal with GPIOs. This patch makes GPIO_DEVRES available
only if GPIOLIB is selected.

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