MAINTAINERS: Add Paul Burton as MIPS co-maintainer

I soon won't have access to much MIPS hardware, nor enough time to
properly maintain MIPS on my own, so add Paul Burton as a co-maintainer.

Also add a link to a new shared git repository on kernel.org for
linux-next branches and pull request tags.

Signed-off-by: James Hogan <jhogan@kernel.org>
Acked-by: Paul Burton <paul.burton@mips.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Matt Redfearn <matt.redfearn@mips.com>
Cc: Maciej W. Rozycki <macro@mips.com>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/19473/