Update the RISC-V MAINTAINERS file

Now that we're upstream in Linux we've been able to make some
infrastructure changes so our port works a bit more like other ports.
Specifically:

* We now have a mailing list specific to the RISC-V Linux port, hosted
  at lists.infreadead.org.
* We now have a kernel.org git tree where work on our port is
  coordinated.

This patch changes the RISC-V maintainers entry to reflect these new
bits of infrastructure.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>