Previously the uarts attached in order, so that the first attached uart

Previously the uarts attached in order, so that the first attached uart
is always the serial console.  With device tree enumeration, this order
is not given anymore.  The serial console might now attach with a
different minor and has to be updated, otherwise there will be no
further output on the serial.  Thus, re-create the i.MX6 console with
the correct minor number on attach.

ok jsg@, kettenis@