doc: rockchip: Add doc for rk3399 TPL build/flash

This patch add documentation for TPL build and flashing steps
for rk3399 boards.

Add full boot log for future reference.

Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
1 file changed