m68knommu: clean up use of MBAR for DRAM registers on ColdFire start

In some of the RAM size autodetection code on ColdFire CPU startup
we reference DRAM registers relative to the MBAR register. Not all of
the supported ColdFire CPUs have an MBAR, and currently this works
because we fake an MBAR address on those registers. In an effort to
clean this up, and eventually remove the fake MBAR setting make the
DRAM register address definitions actually contain the MBAR (or IPSBAR
as appropriate) value as required.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>