soc: arm: Enable mcux flexcan driver on i.mx rt socs

Enables the mcux flexcan driver on i.mx rt socs by default when
CONFIG_CAN=y.

This fixes a runtime failure in tests/subsys/canbus/isotp/conformance on
the mimxrt1064_evk board:

Assertion failed at WEST_TOPDIR/zephyr/tests/subsys/canbus/isotp/conformance/src/main.c:883: test_main: (can_dev is NULL)
    CAN device not not found

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>