m68k: Select M68020 as fallback for classic

Building without a CPU selected does not work.

Change the Kconfig logic slightly to make sure we always pick M68020 if
nothing else is enabled.  There are still some intentional warnings for
builds with all platforms disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/20241030195638.22542-2-arnd@kernel.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

Author: arndb

arch/m68k/Kconfig.cpu

@@ -21,6 +21,7 @@ choice
 config M68KCLASSIC
 	bool "Classic M68K CPU/machine family support"
 	select HAVE_ARCH_PFN_VALID
+	select M68020 if MMU && !(M68030 || M68040 || M68060)
 
 config COLDFIRE
 	bool "Coldfire CPU family support"