pinctrl: freescale: make mxs_regs const

Make these const as they are only stored in the const field of a
mxs_pinctrl_soc_data structure.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>