m68knommu: add basic mmu-less m548x support

Add a very basic mmu-less support for coldfire m548x family.  This is perhaps
also valid for m547x family.  The port comprises the serial, tick timer and
reboot support.  The gpio part compiles but is empty.  This gives a functional
albeit limited linux for the m548x coldfire family.  This has been tested
on a Freescale M548xEVB Lite board with a M5484 processor and the default
dbug monitor.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>