doc: board: ti: Optimize sources references

We have duplication of sources which makes it hard to sustain across the
board, but at the same time, we'd like to ensure readers get specific
information without having to cross refer to different documentation to
get piecemeal information that they need to put together.

Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Nishanth Menon <nm@ti.com>