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

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/[email protected]/
Reported-by: Simon Glass <[email protected]>
Suggested-by: Mattijs Korpershoek <[email protected]>
Signed-off-by: Nishanth Menon <[email protected]>
Signed-off-by: Heinrich Schuchardt <[email protected]>