can: flexcan: adopt pinctrl support

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