README: Update links to mailing list archives

README: Update links to mailing list archives

This patch includes these updates

- Change http://lists.denx.de/pipermail/u-boot
  to https://lists.denx.de/pipermail/u-boot
- http://dir.gmane.org/gmane.comp.boot-loaders.u-boot
  is broken, so remove it
- Another archive https://marc.info/?l=u-boot
  is available, so add it

Additional information:
dir.gmane.org has moved to news.gmane.io, but it only allows
access to the archive via NNTP.

- Server: news.gmane.io
- Group : gmane.comp.boot-loaders.u-boot

I don't include this information in this patch because
it's not very useful without web interfaces.

Signed-off-by: Naoki Hayama <naoki.hayama@lineo.co.jp>