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>

MAINTAINERS

@@ -13485,7 +13485,7 @@ RISC-V ARCHITECTURE
 M:	Palmer Dabbelt <palmer@sifive.com>
 M:	Albert Ou <aou@eecs.berkeley.edu>
 L:	linux-riscv@lists.infradead.org
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/palmer/riscv-linux.git
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git
 S:	Supported
 F:	arch/riscv/
 K:	riscv