doc: board: ti: k3: Convert to sphinx-prompt

Sphinx-prompt provides a handy scheme to provide documentation that
renders nicely and yet provides a scheme to copy paste for users without
having to hand-edit the copied text as is the result of code-block

[1] https://lore.kernel.org/all/87fs48rgto.fsf@baylibre.com/


Reported-by: Simon Glass <sjg@chromium.org>
Suggested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>