pinctrl: airoha: replace struct function_desc with struct pinfunction

pinctrl: airoha: replace struct function_desc with struct pinfunction

struct function_desc is a wrapper around struct pinfunction with an
additional void *data pointer. This driver doesn't use the data pointer.
We're also working towards reducing the usage of struct function_desc in
pinctrl drivers - they should only be created by pinmux core and
accessed by drivers using pinmux_generic_get_function(). Replace the
struct function_desc objects in this driver with smaller struct
pinfunction instances.

Tested-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>