scripts: get_abi.pl: auto-generate cross references

scripts: get_abi.pl: auto-generate cross references

There are several cross-references that can be automatically
generated:

	- References to .rst files inside Documentation/
	- References to other ABI files;
	- References to ABI symbols at /sys/*.

Add a logic to automatically parse them and convert into
cross references.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/abe756d4f94fb6ffcc3dd3902a766c7c3990ea89.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>