can: flexcan: Use a regulator to control the CAN transceiver

Instead of using a GPIO to turn on/off the CAN transceiver, it is better to
use a regulator as some systems may use a PMIC to power the CAN transceiver.

Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>