rockchip: evb_rk3229: Update/fix README

This updates the evb_rk3229's README on howto create / use the FIT image
created by binman.
Also fix some wrong paths and update filenames which have changed in recent
upstream optee-os versions.

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>