pinctrl: sh-pfc: Constify enum_ids and var_field_width compound literals

The enum_ids and var_field_width fields of struct pinmux_data_reg and
pinmux_cfg_reg are initialized using compound literals. Cast them to
const to store them in .rodata.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>