scripts/get_abi.pl: avoid use literal blocks when not needed

The usage of literal blocks make the document very complex,
causing the browser to take a long time to load.

On most ABI descriptions, they're a plain text, and don't
require a literal block.

So, add a logic there with identifies when a literal block
is needed.

As, on literal blocks, we need to respect the original
document space, the most complex part of this patch is
to preserve the original spacing where needed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>