rockchip: rk3368: Add core start-up code for RK3368

The RK3368 is an octa-core Cortex-A53 SoC from Rockchip.
This adds basic support to chain-load U-Boot from Rockchip's
miniloader.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>