MAINTAINERS: add RISC-V's patchwork

MAINTAINERS: add RISC-V's patchwork

The RISC-V patchwork instance on kernel.org has had some necromancy
performed on it & will be used going forward. The statuses that are
intended to be used are:
- New: No action has been taken yet
- Under Review: The maintainer is waiting for review comments from others
- Changes Requested: Either the maintainer or a reviewer requested
  changes in the patch. The patch author is expected to submit a new
  version
- Superseded: There's a new version of the patch available
- Not Applicable: The patch is not intended for the RISC-V tree
- Accepted: The patch has been applied
- Rejected: The patch has been rejected, with reasons stated in an
  email

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221011160744.2167025-1-conor@kernel.org/
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>