Add STDIO documentation template.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
[trini: Fix DOCBOOKS line as we don't have linker_lists yet]
Signed-off-by: Tom Rini <trini@ti.com>