rockchip: Implement spl_gpio in the GPIO driver

Allow rockchip boards to use GPIOs before driver model is ready. This is
really only useful for setting GPIOs to enable the early debug console, if
needed on some platforms.

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