pinctrl: pinmux: open-code PINCTRL_FUNCTION_DESC()

pinctrl: pinmux: open-code PINCTRL_FUNCTION_DESC()

This macro is only used in one place and pin function descriptors should
only be created by pinmux core so there's no point in exposing it to
other pinctrl users. Remove the macro and hand-code its functionality.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/20250709-pinctrl-gpio-pinfuncs-v2-1-b6135149c0d9@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>