boards: arm: mimxrt1060_evk: switch to using FlexCAN3

Switch the NXP i.MX RT1060 evaluation kit from using FlexCAN2 to FlexCAN3.

FlexCAN1 and FlexCAN2 are standard FlexCAN controllers supporting CAN 2.0b,
whereas FlexCAN3 supports both CAN 2.0b and CAN-FD. Switching to FlexCAN3
by default allows testing the CAN-FD capabilities of this board.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>