rockchip: rk3188: Add Radxa Rock board

The Rock is a RK3188 based single board computer by Radxa.
Currently it still relies on the proprietary DDR init and
cannot use the generic SPL, but at least is able to boot
a linux kernel and system up to a regular login prompt.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Fix sort order in defconfig, enable CONFIG_SPL_TINY_MEMSET:
Signed-off-by: Simon Glass <sjg@chromium.org>