m68k: Move Sun 3 into a top-level platform option

m68k: Move Sun 3 into a top-level platform option

It is possible to select an m68k MMU build but not actually enable any
of the three MMU options, which then results in a build failure:

    arch/m68k/include/asm/page.h:10:25: error: 'CONFIG_PAGE_SHIFT' undeclared here (not in a function); did you mean 'CONFIG_LOG_BUF_SHIFT'?

Change the Kconfig selection to ensure that exactly one of the three
options is always enabled whenever an MMU-enabled kernel is built, but
moving CONFIG_SUN3 into a top-level option next to M68KCLASSIC and
COLDFIRE.

All defconfig files should keep working without changes, but
alldefconfig now builds support for the classic MMU.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202408032138.P7sBvIns-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/20241030195638.22542-1-arnd@kernel.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>