pinctrl: pinctrl-at91-pio4: PM related attribute descriptions

pinctrl: pinctrl-at91-pio4: PM related attribute descriptions

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

 drivers/pinctrl/pinctrl-at91-pio4.c:132: warning: Function parameter or member 'pm_wakeup_sources' not described in 'atmel_pioctrl'
 drivers/pinctrl/pinctrl-at91-pio4.c:132: warning: Function parameter or member 'pm_suspend_backup' not described in 'atmel_pioctrl'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200713144930.1034632-16-lee.jones@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>