pinctrl: nomadik: nmk_prcm_gpiocr_get_mode may be unused

nmk_prcm_gpiocr_get_mode is only needed for debugfs output at
the moment, which can be compile-time disabled. Marking
the function __maybe_unused still gives us compile-time
coverage, but avoids a gcc warning.

Without this patch, building nhk8815_defconfig results in:

drivers/pinctrl/pinctrl-nomadik.c:676:12: warning: 'nmk_prcm_gpiocr_get_mode' defined but not used [-Wunused-function]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Jean-Nicolas Graux <jean-nicolas.graux@stericsson.com>
Cc: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>