doc: boards: Add rockchip documentation

Rockchip has documentation file, doc/README.rockchip but
which is not so readable to add or understand the existing
contents. Even the format that support is legacy readme
in U-Boot.

Add rockchip specific documentation file using new rst
format, which describes the information about Rockchip
supported boards and it's usage steps.

Added minimal information about rk3288, rk3328, rk3368
and rk3399 boards and usage. This would indeed updated
further based on the requirements and updates.

Cc: Kever Yang <kever.yang@rock-chips.com>
Cc: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>