pinctrl: imx: Allow parsing DT without function nodes

The old format to define pinctrl settings for imx in DT has two
hierarchy levels. The first level are function device nodes. The second
level are pingroups which contain a property fsl,pins. The original
intention was to define all pin functions in a single dtsi file and just
reference the correct ones in the board files.
This idea was rejected some time ago leading to the current design to
have all the pinfunctions defined in the board files. So we don't need
the function device nodes anymore.

This patch changes the pinctrl driver to accept devicetrees which do not
have the first hierarchy level, function device nodes. For example
karo-tx25 already has such a devicetree. Old devicetrees are still
parsed and supported.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>