net: flexcan: clock-frequency is optional for device tree probe

The property clock-frequency is optional for device tree probe.  When
it's absent, the flexcan driver will try to get the frequency from clk
system by calling clk_get_rate.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

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

@@ -11,6 +11,9 @@ Required properties:
 
 - reg : Offset and length of the register set for this device
 - interrupts : Interrupt tuple for this device
+
+Optional properties:
+
 - clock-frequency : The oscillator frequency driving the flexcan device
 
 Example: