m68k: fix ColdFire PCI config reads and writes

The ColdFire PCI configuration space access functions swap addressing
regions to do their work. Just letting the read/write cycles exit
the CPU core (via the ColdFire "nop" instruction) is not enough to
guarantee that the address region remapping has actually completed.
Insert a read back of the mapping register to be absolutely sure
that the remapping has completed.

This fixes an occasional boot hang during the ColdFire PCI initialization
phase.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Reviewed-by: Angelo Dureghello <angelo@sysam.it>
Tested-by: Angelo Dureghello <angelo@sysam.it>