ARM: at91: pm: Move prototypes to mutually included header

Both the caller and the supplier's source file should have access to
the include file containing the prototypes.

Fixes the following W=1 kernel build warning(s):

 drivers/pinctrl/pinctrl-at91.c:1637:6: warning: no previous prototype for ‘at91_pinctrl_gpio_suspend’ [-Wmissing-prototypes]
 1637 | void at91_pinctrl_gpio_suspend(void)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/pinctrl/pinctrl-at91.c:1661:6: warning: no previous prototype for ‘at91_pinctrl_gpio_resume’ [-Wmissing-prototypes]
 1661 | void at91_pinctrl_gpio_resume(void)
 | ^~~~~~~~~~~~~~~~~~~~~~~~

Cc: Russell King <linux@armlinux.org.uk>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210303124149.3149511-1-lee.jones@linaro.org