Add PINCTRL_FUNCTION_DESC() macro for inline use.

While at it, fix adjective form in the comment of PINCTRL_GROUP_DESC().
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240530085745.1539925-6-andy.shevchenko@gmail.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>