MAINTAINERS: SiFive drivers: change the git tree to a SiFive git tree

Update the git tree URL for drivers for SiFive-related IP blocks to
point to a SiFive-managed URL.

Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>