Allow Serial pin to be NC on Freescale/NXP devices

Nordic(?)/STM should also be done, but I don't know if they do their own
stuff.

Issue is that since the mbed-assert implementation, calling pin_mode
causes an assert when a pin is NC. Since defining a serial object with
only TX or RX is a valid use case, this should be handled.

pinmap_pinout does accept NC pins, so there no guards are needed (person
who adds asserts there may also fix it in all other code).