pinctrl: imx: use generic pinmux helpers for managing functions

Now using function_desc structure instead of imx_pmx_func.

Also leveraging generic functions to retrieve functions count/name/groups.

The imx_free_funcs function can be removed since it is now handled by
the core driver during unregister.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>