gpio: uapi: use the preferred SPDX license identifier

GPL-2.0 license identifier is deprecated. User-space projects that want
to include the kernel header with their source-code will be unable to
become fully REUSE compliant due to the reuse tool complaining about
deprecated licenses. Change the SPDX identifier to GPL-2.0-only.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>