dt-bindings: net: Document Broadcom SYSTEMPORT clocks

The Broadcom SYSTEMPORT adapters require the use of two clocks for
normal operations and during Wake-on-LAN, document those in the binding
document.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>