MAINTAINERS: Add Paul Burton as MIPS co-maintainer

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 <[email protected]>
Acked-by: Paul Burton <[email protected]>
Acked-by: Florian Fainelli <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Matt Redfearn <[email protected]>
Cc: Maciej W. Rozycki <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: Aaro Koskinen <[email protected]>
Cc: John Crispin <[email protected]>
Cc: Steven J. Hill <[email protected]>
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/19473/