pinctrl: remove useless const qualifier

This "const" claims the get_function_groups callback never
changes the given num_groups pointer.  It is always true
in C language, so not worth mentioning.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>