gpio: Add a simple GPIO API for SPL

In space-constrained environments or before driver model is available, it
is sometimes necessary to set GPIO values. Add an SPL API for this, to
allow early board code to change GPIOs. The caller must provide the
register address, so that the drivers can be fairly generic.

This API can be implemented by GPIO drivers, behind a suitable guard,
like #ifdef CONFIG_SPL_BUILD.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>