pinctrl: pinmux: Embed struct pinfunction into struct function_desc

struct function_desc is a particular version of the struct pinfunction
with associated opaque data. Start switching pin control core and
drivers to use it explicitly.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240530085745.1539925-7-andy.shevchenko@gmail.com


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