m68k: Allow kexec on M68KCLASSIC with MMU enabled only

m68k: Allow kexec on M68KCLASSIC with MMU enabled only

None of the no-MMU CPU types (classic or ColdFire) or ColdFire with
MMU enabled have support for kexec yet, so the configuration as it
stands is not quite right, and it will fail to compile on them.

Make CONFIG_KEXEC depend on both CONFIG_M68KCLASSIC and CONFIG_MMU.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20220831012539.3416470-1-gerg@linux-m68k.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>