doc: ti: update bash code-block directives to prompt

doc: ti: update bash code-block directives to prompt

The code-block directive requires addition of the prompt symbol for each
line, using the prompt directive instead allows for auto insertion of
the symbol per line[1].

For the readers, the character added by the prompt directive is
un-selectable i.e the entire line can be more easily selected for copy
pasting etc. Whereas with code-block, the prompt symbol like "$" is also
selectable which is usually not the intent.

This is mostly a QoL addition + making the docs consistent since k3.rst
makes use of prompt directives which these board docs include from.

[1]: https://pypi.org/project/sphinx-prompt/

Signed-off-by: Anshul Dalal <anshuld@ti.com>