pinctrl: add one more "const" for generic function groups

Generic code doesn't modify those strings and .get_function_groups
callback has that extra "const" as well. This allows more flexibility in
GENERIC_PINMUX_FUNCTIONS users.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20211216162206.8027-4-zajec5@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>