dt-bindings: net: document ptp_ref clk in dwmac

ptp_ref clk has been added in DT but not documented which makes yaml
validation failed:

ethernet@5800a000: clocks: [[6, 105], [6, 103], [6, 104], [6, 123],
[6, 169], [6, 112]] is too long
ethernet@5800a000: clock-names: ['stmmaceth', 'mac-clk-tx', 'mac-clk-rx',
'eth-ck', 'ptp_ref', 'ethstp'] is too long

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>