pinctrl: devicetree: Use strlen() instead of hardcoded number

Improve readability by replacing a hardcoded number requiring a comment
by strlen().

Gcc is smart enough to evaluate the length of a constant string at
compile-time.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190731132917.17607-2-geert+renesas@glider.be


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>