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

Since rockchip have an individual doc/README.rockchip, it would
be better to update the same instead of maintaining it separately
in board files.

So, add the documentation for rk3399
- procedure to build for Rockchip miniloader and
  U-Boot SPL options
- procedure to boot from SD for Rockchip miniloader and
  U-Boot SPL options
- procedure to build ATF, PMU M0 firmware for puma boards
- add boot logs of each option, so-that it would help for future
  boards porting

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>