dt-bindings: net: dsa: sf2: fix brcm,use-bcm-hdr documentation

The property use-bcm-hdr was documented as an entry under the ports node
for the bcm_sf2 DSA switch. This property is actually evaluated for each
port. Correct the documentation to match the actual behavior and properly
reference dsa-port.yaml for additional properties of the node.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>