gpio: Add a GPIO config

gpio: Add a GPIO config

At present we have SPL_GPIO and TPL_GPIO but not piain GPIO. This
works because there is a special build rule in Makefile that always
includes the drivers/gpio directory.

It is better to have all driver directories included by drivers/Makefile
and there is already a rule in there for this purpose. It just needs a
Kconfig for U-Boot proper, so add one.

Enable the option always for now, since this mimics current behaviour.
This can be updated once DM_GPIO is used everywhere.

Signed-off-by: Simon Glass <sjg@chromium.org>