sfp: add documentation for kernel APIs

Add kernel-doc documentation for sfp kernel APIs, and link it into the
networking kapi documentation under "Network device support".

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