gpio: nomadik: add #include <linux/slab.h>

Add linux/slab.h header include for GFP flags.

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://lore.kernel.org/r/20240228-mbly-gpio-v2-14-3ba757474006@bootlin.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>