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.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>