pinctrl: imx: use generic pinctrl helpers for managing groups

Now using group_desc structure instead of imx_pin_group.

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

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

Finally the device tree parsing is moved after the pinctrl driver registration
since this latter initializes the radix trees.

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