gpio: tps65xxx: Use SPDX license tag

I'm tired of boilerplate, use the SPDX tag.

Acked-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>