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>