stmmac: Correct documentation on stmmac clocks.

devm_get_clk looks in clock-name property for matching clock.
the ptp_ref_clk property is ignored.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>