dt/bindings: add clock-select function property to micrel phy binding

Add "micrel,rmii-reference-clock-select-25-mhz" to Micrel ethernet PHY
binding documentation.

This property is needed to properly describe some revisions of Micrel
PHYs which has the function of this configuration bit inverted so that
setting it enables 25 MHz rather than 50 MHz clock mode.

Note that a clock reference ("rmii-ref") is still needed to actually
select either mode.

Cc: devicetree@vger.kernel.org
Signed-off-by: NJohan Hovold <johan@kernel.org>
Acked-by: NSascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/micrel.txt

@@ -19,6 +19,17 @@ Optional properties:
 
 	See the respective PHY datasheet for the mode values.
 
+ - micrel,rmii-reference-clock-select-25-mhz: RMII Reference Clock Select
+						bit selects 25 MHz mode
+
+	Setting the RMII Reference Clock Select bit enables 25 MHz rather
+	than 50 MHz clock mode.
+
+	Note that this option in only needed for certain PHY revisions with a
+	non-standard, inverted function of this configuration bit.
+	Specifically, a clock reference ("rmii-ref" below) is always needed to
+	actually select a mode.
+
  - clocks, clock-names: contains clocks according to the common clock bindings.
 
 	supported clocks: