can: flexcan: add support for PE clock source select

Add support to select the clock source for CAN Protocol Engine (PE).
It's SoC Implementation dependent. Refer to RM for detailed definition
of each SoC. We select clock source 1 (peripheral clock) by default in
driver now, this patch adds support to parse the clock source from the DT.

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>