MAINTAINERS: Update linux-mips mailing list address

The linux-mips.org infrastructure has been unreliable recently & nobody
with sufficient access to fix it is around to do so. As a result we're
moving away from it, and part of this is migrating our mailing list to
kernel.org.

Replace all instances of linux-mips@linux-mips.org in MAINTAINERS with
the shiny new linux-mips@vger.kernel.org address.

The new list is now being archived on kernel.org at
https://lore.kernel.org/linux-mips/ which also holds the history of the
old linux-mips.org list.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: linux-mips@linux-mips.org