Documentation: devicetree: net: refer to fixed-link.txt

Update the Freescale TSEC PHY, Broadcom GENET & SYSTEMPORT Device Tree
binding documentation to refer to the fixed-link Device Tree binding in
fixed-link.txt.

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>