dt-bindings: phy: dp83867: Add documentation for disabling clock output

The clock output is generally only used for testing and development and
not used to daisy-chain PHYs.  It's just a source of RF noise afterward.

Add a mux value for "off".  I've added it as another enumeration to the
output property.  In the actual PHY, the mux and the output enable are
independently controllable.  However, it doesn't seem useful to be able
to describe the mux setting when the output is disabled.

Document that PHY's default setting will be left as is if the property
is omitted.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ti,dp83867.txt

@@ -33,8 +33,10 @@ Optional property:
 				    software needs to take when this pin is
 				    strapped in these modes. See data manual
 				    for details.
-	- ti,clk-output-sel - Muxing option for CLK_OUT pin - see dt-bindings/net/ti-dp83867.h
-				    for applicable values.
+	- ti,clk-output-sel - Muxing option for CLK_OUT pin.  See dt-bindings/net/ti-dp83867.h
+			      for applicable values.  The CLK_OUT pin can also
+			      be disabled by this property.  When omitted, the
+			      PHY's default will be left as is.
 
 Note: ti,min-output-impedance and ti,max-output-impedance are mutually
       exclusive. When both properties are present ti,max-output-impedance

include/dt-bindings/net/ti-dp83867.h

@@ -56,4 +56,6 @@
 #define DP83867_CLK_O_SEL_CHN_C_TCLK		0xA
 #define DP83867_CLK_O_SEL_CHN_D_TCLK		0xB
 #define DP83867_CLK_O_SEL_REF_CLK		0xC
+/* Special flag to indicate clock should be off */
+#define DP83867_CLK_O_SEL_OFF			0xFFFFFFFF
 #endif