MAINTAINERS: Add sonet.h to ATM section of MAINTAINERS

This is part of an effort to assign a section in MAINTAINERS to header
files that relate to Networking. In this case the files with "net" in
their name.

It seems that sonet.h is included in ATM related source files,
and thus that ATM is the most relevant section for these files.

Cc: Chas Williams <3chas3@gmail.com>
Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>