MAINTAINERS: don't automatically patches involving SiFive to the linux-riscv list

The current K: entry in the "SIFIVE DRIVERS" section causes
scripts/get_maintainer.pl to recommend that all patches that originate
from, or are sent or copied to, anyone with a @sifive.com E-mail
address to be copied to the linux-riscv@lists.infradead.org mailing
list:

https://lore.kernel.org/linux-riscv/CABEDWGxKCqCq2HBU8u1-=QgmMCdb69oXxN5rz65nxNODxdCAnw@mail.gmail.com/

This is undesirable, since not all of these patches may be relevant to
the linux-riscv@ mailing list.  Fix by excluding K: matches that look
like a sifive.com E-mail address.

Based on the following patch from Palmer Dabbelt <palmer@sifive.com>:

https://lore.kernel.org/linux-riscv/mhng-2a897a66-1f3d-4878-ba47-1ae36b555540@palmer-si-x1e/



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