docs: sphinx/kernel_abi: parse ABI files only once

docs: sphinx/kernel_abi: parse ABI files only once

Right now, the logic parses ABI files on 4 steps, one for each
directory. While this is fine in principle, by doing that, not
all symbol cross-references will be created.

Change the logic to do the parsing only once in order to get
a global dictionary to be used when creating ABI cross-references.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/5205c53838b6ea25f4cdd4cc1e3d17c0141e75a6.1739182025.git.mchehab+huawei@kernel.org