gpio: wm8xxx: Cut down on boilerplate

Just use the SPDX license tag for these drivers.

Cc: patches@opensource.cirrus.com
Cc: Charles Keepax <ckeepax@opensource.cirrus.com>
Cc: Mark Brown <broonie@kernel.org>
Acked-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>