dt-bindings: add sff,sff binding for SFP support

Add "sff,sff" for SFF module support with SFP.  These have a different
phys_id value, and also have the present and rate select signals omitted
compared with their socketed counter-parts.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>