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>