MAINTAINERS: change the arch/riscv git tree to the new shared tree

Palmer, with Konstantin's gracious help, set up a shared kernel.org
git tree for arch/riscv patches going forward.  Change the MAINTAINERS
file accordingly.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>