m68k: fix early memory reservation for ColdFire MMU systems

The bootmem to memblock conversion introduced by the commit 1008a115
("m68k: switch to MEMBLOCK + NO_BOOTMEM") made reservation of kernel code
and data to start from a wrong address.

Fix it.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Tested-by: Angelo Dureghello <angelo@sysam.it>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>