[U-Boot] [U-Boot, v3, 10/11] rockchip: evb-rk3229: add README file for OP-TEE support

[Philipp Tomsich]

> Detail of step by step to bring up the board with OP-TEE support.
> 
> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
> Changes in v3: None
> Changes in v2: None
> 
>  board/rockchip/evb_rk3229/README | 72 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 72 insertions(+)
>  create mode 100644 board/rockchip/evb_rk3229/README
> 

Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>