pinctrl: ingenic: Provide a helper macro INGENIC_PIN_FUNCTION()

Provide a helper macro to assign the struct function_desc entries.
This helps further refactoring.
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240530085745.1539925-4-andy.shevchenko@gmail.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>