doc: board: ti: Update documentation for binman flow

Earlier documentation specified builds for generating bootloader images
using an external TI repository k3-image-gen and core-secdev-k3. Modify
this to using the binman flow so that user understands how to build the
final boot images.

Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>