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 <[e-mail]>
Reviewed-by: Martin Kepplinger <[e-mail]>
Acked-by: Federico Vaga <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/input/devices/xpad.rst

···
 .. [1] http://euc.jp/periphs/xbox-controller.ja.html (ITO Takayuki)
 .. [2] http://xpad.xbox-scene.com/
 .. [3] http://www.markosweb.com/www/xboxhackz.com/
-.. [4] http://lxr.free-electrons.com/ident?i=xpad_device
+.. [4] https://elixir.bootlin.com/linux/latest/ident/xpad_device
 
 
 Historic Edits

Documentation/process/howto.rst

···
 self-referential, indexed webpage format. An excellent up-to-date
 repository of the kernel code may be found at:
 
-	http://lxr.free-electrons.com/
+	https://elixir.bootlin.com/
 
 
 The development process

Documentation/process/kernel-docs.rst

···
 
     * Name: **Cross-Referencing Linux**
 
-      :URL: http://lxr.free-electrons.com/
+      :URL: https://elixir.bootlin.com/
       :Keywords: Browsing source code.
       :Description: Another web-based Linux kernel source code browser.
         Lots of cross references to variables and functions. You can see

Documentation/translations/it_IT/process/howto.rst

···
 sorgente in un formato autoreferenziale ed indicizzato. Un eccellente ed
 aggiornata fonte di consultazione del codice del kernel la potete trovare qui:
 
-	http://lxr.free-electrons.com/
+	https://elixir.bootlin.com/
 
 
 Il processo di sviluppo

Documentation/translations/ja_JP/howto.rst

···
 できます。この最新の素晴しいカーネルコードのリポジトリは以下で見つかり
 ます -
 
-	http://lxr.free-electrons.com/
+	https://elixir.bootlin.com/
 
 開発プロセス
 ------------

Documentation/translations/ko_KR/howto.rst

···
 소스코드를 인덱스된 웹 페이지들의 형태로 보여준다. 최신의 멋진 커널
 코드 저장소는 다음을 통하여 참조할 수 있다.
 
-      http://lxr.free-electrons.com/
+      https://elixir.bootlin.com/
 
 
 개발 프로세스