cpsw: add a DT field for the active time stamping port

Because time stamping on both external ports of the switch simultaneously
is positively useless from the application's point of view, this patch
provides a DT configuration method to choose the active port.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/cpsw.txt

@@ -23,6 +23,7 @@ Required properties:
 - mac_control		: Specifies Default MAC control register content
 			  for the specific platform
 - slaves		: Specifies number for slaves
+- cpts_active_slave	: Specifies the slave to use for time stamping
 - slave_reg_ofs		: Specifies slave register offset
 - sliver_reg_ofs	: Specifies slave sliver register offset
 - phy_id		: Specifies slave phy id
@@ -60,6 +61,7 @@ Examples:
 		rx_descs = <64>;
 		mac_control = <0x20>;
 		slaves = <2>;
+		cpts_active_slave = <0>;
 		cpsw_emac0: slave@0 {
 			slave_reg_ofs = <0x200>;
 			sliver_reg_ofs = <0xd80>;
@@ -95,6 +97,7 @@ Examples:
 		rx_descs = <64>;
 		mac_control = <0x20>;
 		slaves = <2>;
+		cpts_active_slave = <0>;
 		cpsw_emac0: slave@0 {
 			slave_reg_ofs = <0x200>;
 			sliver_reg_ofs = <0xd80>;