MAINTAINERS: Add entry for RV32G BPF JIT

Add a new entry for the 32-bit RISC-V JIT to MAINTAINERS and change
mailing list to netdev and bpf following the guidelines from
commit e42da4c6 ("docs/bpf: Update bpf development Q/A file").

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Björn Töpel <bjorn.topel@gmail.com>
Acked-by: Björn Töpel <bjorn.topel@gmail.com>
Link: https://lore.kernel.org/bpf/20200305050207.4159-5-luke.r.nels@gmail.com