doc: rockchip: add px30/rk3326 boards and examples

There are several PX30/RK3326 boards in use without
mentioning in rockchip.rst. Add boards and examples.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
1 file changed