scripts: get_abi.pl: parse description line per line

Change the description parsing logic in rst mode in order
to parse it line per line.

The end result is the same, but doing line per line allows
to add some code to escape literal blocks when seeking for
cross-references.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d33cfa2e59ecf8f28d4ed7de7402468cf2168921.1616668017.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>