pinctrl: imx: use radix trees for groups and functions

This change is inspired from the pinctrl-single architecture.

The problem with current implementation is that it isn't possible
to add/remove functions and/or groups dynamically. The radix tree
offers an easy way to do so. The intent is to offer a follow-up
patch later that will enable the use of pinctrl nodes in dt-overlays.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>