dt-bindings: net: convert sff,sfp to dtschema

Convert the sff,sfp.txt bindings to the DT schema format.
Also add the new path to the list of maintained files.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>