pinctrl: imx: prepare for making "group_names" in "function_desc" const

The plan for "struct function_desc" is to make its "group_names"
/double/ const. That will allow drivers to use it with static const
data.

This imx change is required to avoid:
drivers/pinctrl/freescale/pinctrl-imx.c: In function 'imx_pinctrl_parse_functions':
drivers/pinctrl/freescale/pinctrl-imx.c:672:24: error: assignment of read-only location '*(func->group_names + (sizetype)(i * 4))'
  672 |   func->group_names[i] = child->name;
      |                        ^

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20211216162206.8027-1-zajec5@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>