pinctrl: Minimize SPDX hamming distance

OK so some automatic scripts were fixing the SPDX tags in
the mainline branch while we were patching other stuff,
and yeah it is more correct to have "GPL-2.0-only" rather
than "GPL-2.0" so let's conform to what is already upstream
so we don't end up getting the wrong license on the merged
result later.

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