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 <j.neuschaefer@gmx.net>
Reviewed-by: Martin Kepplinger <martin.kepplinger@ginzinger.com>
Acked-by: Federico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>