m68knommu: make ColdFire internal peripheral region configurable

Most ColdFire CPUs have an internal peripheral set that can be mapped at
a user selectable address. Different ColdFire parts either use an MBAR
register of an IPSBAR register to map the peripheral region. Most boards
use the Freescale default mappings - but not all.

Make the setting of the MBAR or IPSBAR register configurable. And only make
the selection available on the appropriate ColdFire CPU types.

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