pinctrl: at91: use __maybe_unused to hide pm functions

The at91-pio4 pinctrl driver uses SET_SYSTEM_SLEEP_PM_OPS() to
conditionally set the correct suspend/resume options, but they
become unused when CONFIG_PM is disabled:

drivers/pinctrl/pinctrl-at91-pio4.c:827:12: error: 'atmel_pctrl_suspend' defined but not used [-Werror=unused-function]
drivers/pinctrl/pinctrl-at91-pio4.c:847:12: error: 'atmel_pctrl_resume' defined but not used [-Werror=unused-function]

This adds __maybe_unused annotations so the compiler knows
it can silently drop them instead of warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>