m68knommu: improve config ROM setting defaults

The ROM region configuration settings used on some nommu m68k systems
(historically mostly 68328 (Dragonball) CPUs) default to an address
of 0. That can easily clash with default RAM address settings which
also default to 0. Of course that is invalid and those ranges overlap,
but if you make no value selection that is what you end up with. Those
default values produce a valid configuration but will fail compilation
like this:

    m68k-linux-ld: section .rodata VMA [0000000000001000,0000000000262227] overlaps section .text VMA [0000000000000400,0000000000455e7f]

Looking at the platforms that use the ROM region configuration settings
it is clear that we can choose much better defaults than 0. By far the
most common ROM region settings are these:

    CONFIG_ROMVEC=0x10c10000
    CONFIG_ROMSTART=0x10c10400

So lets make these the default values.

It is still possible to configure overlapping ROM and RAM regions, but
at least the default selections are now valid.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202305301407.z33zOjcG-lkp@intel.com/


Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>