pinctrl: mxs: warn if functions are not grouped by name

The mxs pinctrl driver cannot handle when functions are not grouped by
name (which IMO is a bug). This happens for example if a
imx28-somemachine.dts provides a function that has the same name as a
function defined in imx28.dtsi.

The proper way to fix that would be to check for duplicates in the loops
(which increases parsing time) or parse the groups first and sort the
resulting array.

Signed-off-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>