pinctrl: use "const struct ..." rather than "struct ... const"

Only this member, pins, is defined as "struct ... const *", but the
others in this struct, pinlops, pmxops, confops, etc. are defined as
"const struct ... *".

Swap the "struct pinctrl_pin_desc" and "const" for consistency.

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