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 <[e-mail]>
Cc: Ludovic Desroches <[e-mail]>
Cc: Nicolas Ferre <[e-mail]>
Cc: Alexandre Belloni <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

drivers/pinctrl/pinctrl-at91-pio4.c

···
  * @irq_domain: irq domain for the gpio controller.
  * @irqs: table containing the hw irq number of the bank. The index of the
  *     table is the bank id.
+ * @pm_wakeup_sources: bitmap of wakeup sources (lines)
+ * @pm_suspend_backup: backup/restore register values on suspend/resume
  * @dev: device entry for the Atmel PIO controller.
  * @node: node of the Atmel PIO controller.
  */