gpio: make flags mandatory for gpiod_get functions

Now that all[1] users of the gpiod_get functions are converted to make
use of the up to now optional flags parameter, make it mandatory which
allows to remove some cpp magic.

[1] all but etraxfs-uart which is broken anyhow and I'm allowed to
    ignore it by Jesper Nilsson :-)

Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>