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: NGreg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20220831012539.3416470-1-gerg@linux-m68k.orgSigned-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/Kconfig

@@ -87,7 +87,7 @@ config MMU_SUN3
 
 config KEXEC
 	bool "kexec system call"
-	depends on M68KCLASSIC
+	depends on M68KCLASSIC && MMU
 	select KEXEC_CORE
 	help
 	  kexec is a system call that implements the ability to shutdown your