dt-bindings: can: flexcan: add PE clock source property to device tree

The FlexCAN controller can parse clock source property from DTS file to
select PE clock source.
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Documentation/devicetree/bindings/net/can/fsl-flexcan.txt

@@ -32,6 +32,13 @@ Optional properties:
 		 ack_gpr is the gpr register offset of CAN stop acknowledge.
 		 ack_bit is the bit offset of CAN stop acknowledge.
 
+- fsl,clk-source: Select the clock source to the CAN Protocol Engine (PE).
+		  It's SoC Implementation dependent. Refer to RM for detailed
+		  definition. If this property is not set in device tree node
+		  then driver selects clock source 1 by default.
+		  0: clock source 0 (oscillator clock)
+		  1: clock source 1 (peripheral clock)
+
 Example:
 
 	can@1c000 {
@@ -40,4 +47,5 @@ Example:
 		interrupts = <48 0x2>;
 		interrupt-parent = <&mpic>;
 		clock-frequency = <200000000>; // filled in by bootloader
+		fsl,clk-source = <0>; // select clock source 0 for PE
 	};