doc: Change LXR references to elixir.bootlin.com

Recently, Free Electrons was renamed to Bootlin[1]. Less recently, the
Linux Cross Reference (LXR) at lxr.free-electrons.com was replaced by
Elixir[2], and lxr.free-electrons.com redirected first to
elixir.free-electrons.com and now to elixir.bootlin.com.

[1]: https://bootlin.com/blog/free-electrons-becomes-bootlin/
[2]: https://github.com/free-electrons/elixir

Signed-off-by: Jonathan Neuschäfer <[email protected]>
Reviewed-by: Martin Kepplinger <[email protected]>
Acked-by: Federico Vaga <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
6 files changed