rockchip: rk3368: spl: add SPL support

Adds SPL support for the RK3368 (assuming that our TPL stage has
initialised DRAM and set up the memory firewall).

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