pinctrl: imx: remove const qualifier of imx_pinctrl_soc_info

Otherwise can't dynamically update fields such as ngroups which can
change over time (with a dt-overlay for instance).

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>