dt-bindings: can: fsl,flexcan: fix fsl,clk-source property

Correct fsl,clk-source example since flexcan driver uses "of_property_read_u8"
to get this property.

Fixes: 9d733992 ("dt-bindings: can: flexcan: add PE clock source property to device tree")
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Link: https://lore.kernel.org/r/20201106105627.31061-2-qiangqing.zhang@nxp.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>