pinctrl: keembay: use a dedicated structure for the pinfunction description

pinctrl: keembay: use a dedicated structure for the pinfunction description

struct function_desc is a wrapper around struct pinfunction with an
additional void *data pointer. We're 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(). This driver uses the data pointer so in
order to stop using struct function_desc, we need to provide an
alternative that also wraps the mux mode which is passed to pinctrl core
as user data.

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>