gpio: make of_get_named_gpiod_flags() private

gpio: make of_get_named_gpiod_flags() private

of_get_named_gpiod_flags() is visible and directly usable by GPIO
consumers, but it really should not as the gpiod interface relies
on the simpler gpiod_get() to provide properly-configured GPIOs.

of_get_named_gpiod_flags() is just used internally by gpiolib to
implement gpiod_get(), and by the old of_get_named_gpio_flags()
function, therefore it makes sense to make it gpiolib-private.

As a side-effect, the unused (and unneeded) of_get_gpiod_flags()
inline function is also removed, and of_get_named_gpio_flags() is moved
from a static inline function to a regular one in gpiolib-of.c

This results in all references to gpiod_* functions in of_gpio.h being
gone, which is the way it should be since this file is part of the old
integer GPIO interface.

Changes since v1:
- Fixed compilation error when CONFIG_OF_GPIO is not defined
- Fixed warning due to of_gpio_flags enum not being declared
  in private gpiolib.h header

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