of: document external-memory-controller property in tegra124-car

This property contains a phandle to the EMC driver that is needed by the
EMC clock to request the EMC driver to do its part of the clock change
sequence.
Signed-off-by: NTomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: NThierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/clock/nvidia,tegra124-car.txt

@@ -20,6 +20,7 @@ Required properties :
 - #reset-cells : Should be 1.
   In clock consumers, this cell represents the bit number in the CAR's
   array of CLK_RST_CONTROLLER_RST_DEVICES_* registers.
+- nvidia,external-memory-controller : phandle of the EMC driver.
 
 The node should contain a "emc-timings" subnode for each supported RAM type (see
 field RAM_CODE in register PMC_STRAPPING_OPT_A).
@@ -50,6 +51,7 @@ Example SoC include file:
 		reg = <0x60006000 0x1000>;
 		#clock-cells = <1>;
 		#reset-cells = <1>;
+		nvidia,external-memory-controller = <&emc>;
 	};
 
 	usb@c5004000 {