m68k: select CONFIG_GENERIC_ATOMIC64 for all m68k CPU types

There is no specific atomic64 support code for any m68k CPUs, so we should
select CONFIG_GENERIC_ATOMC64 for all. Remove the existing per CPU selection
of this and select it for all m68k.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>