pinctrl: pinctrl-zynqmp: Fix kernel-doc warning

Fix the below kernel-doc warning by adding the description for return
value.
"warning: No description found for return value of
'zynqmp_pmux_get_function_groups'".
Signed-off-by: Sai Krishna Potthuri <lakshmi.sai.krishna.potthuri@xilinx.com>
Link: https://lore.kernel.org/r/1655462819-28801-5-git-send-email-lakshmi.sai.krishna.potthuri@xilinx.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-zynqmp.c

@@ -163,6 +163,8 @@ static const char *zynqmp_pmux_get_function_name(struct pinctrl_dev *pctldev,
  * @num_groups:	Number of function groups.
  *
  * Get function's group count and group names.
+ *
+ * Return: 0
  */
 static int zynqmp_pmux_get_function_groups(struct pinctrl_dev *pctldev,
 					   unsigned int selector,