pinctrl: imx: use struct imx_pinctrl_soc_info as a const

For some SoCs the struct imx_pinctrl_soc_info is passed through
of_device_id.data which is const. Most variables are already const
or otherwise not written. However, some fields are modified at
runtime. Move those fields to the dynamically allocated struct
imx_pinctrl.

Fixes: b3060044 ("pinctrl: freescale: imx7d: make of_device_ids const")
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>