rockchip: Use an external TPL binary on RK3568

Rockchip SoCs typically use U-Boot TPL to initialize DRAM, then jumps
back to BootRom to load next stage, U-Boot SPL, into DRAM. BootRom then
jumps to U-Boot SPL to continue the normal boot flow.

However, there is no support to initialize DRAM on RK35xx SoCs using
U-Boot TPL and instead an external TPL binary must be used to generate a
bootable u-boot-rockchip.bin image.

Add CONFIG_ROCKCHIP_EXTERNAL_TPL to indicate that an external TPL should
be used. Build U-Boot with ROCKCHIP_TPL=/path/to/ddr.bin to generate a
bootable u-boot-rockchip.bin image for RK3568.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Eugen Hristev <eugen.hristev@collabora.com>