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: NRob Herring <robh@kernel.org>
Signed-off-by: NPhil Reid <preid@electromag.com.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/stmmac.txt

@@ -35,15 +35,14 @@ Optional properties:
 - reset-names: Should contain the reset signal name "stmmaceth", if a
 	reset phandle is given
 - max-frame-size: See ethernet.txt file in the same directory
-- clocks: If present, the first clock should be the GMAC main clock and
-  the second clock should be peripheral's register interface clock. Further
-  clocks may be specified in derived bindings.
-- clock-names: One name for each entry in the clocks property, the
-  first one should be "stmmaceth" and the second one should be "pclk".
-- clk_ptp_ref: this is the PTP reference clock; in case of the PTP is
-  available this clock is used for programming the Timestamp Addend Register.
-  If not passed then the system clock will be used and this is fine on some
-  platforms.
+- clocks: If present, the first clock should be the GMAC main clock
+  The optional second clock should be peripheral's register interface clock.
+  The third optional clock should be the ptp reference clock.
+  Further clocks may be specified in derived bindings.
+- clock-names: One name for each entry in the clocks property.
+  The first one should be "stmmaceth".
+  The optional second one should be "pclk".
+  The optional third one should be "clk_ptp_ref".
 - snps,burst_len: The AXI burst lenth value of the AXI BUS MODE register.
 - tx-fifo-depth: See ethernet.txt file in the same directory
 - rx-fifo-depth: See ethernet.txt file in the same directory