dt-bindings: net: xilinx_axienet: Document additional clocks

Update DT bindings to describe all of the clocks that the axienet
driver will now be able to make use of.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>