powerpc/32s: add an option to exclusively select powerpc 601

Powerpc 601 is rather old powerpc which as some important
limitations compared to other book3s/32 powerpcs:
- No Timebase.
- Common BATs for instruction and data.
- No execution protection in segment registers.
- No RI bit in MSR
- ...

It is starting to be difficult and cumbersome to maintain
kernels that are compatible both with 601 and other 6xx cores.

Create a compiletime option to exclusively select either powerpc 601
or other 6xx.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d644eaf7dff8cc149260066802af230bdf34fded.1566834712.git.christophe.leroy@c-s.fr

arch/powerpc/platforms/Kconfig.cputype

@@ -6,6 +6,9 @@ config PPC64
 	  This option selects whether a 32-bit or a 64-bit kernel
 	  will be built.
 
+config PPC_BOOK3S_32
+	bool
+
 menu "Processor support"
 choice
 	prompt "Processor Type"
@@ -21,13 +24,20 @@ choice
 
 	  If unsure, select 52xx/6xx/7xx/74xx/82xx/83xx/86xx.
 
-config PPC_BOOK3S_32
-	bool "512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx"
+config PPC_BOOK3S_6xx
+	bool "512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx except 601"
+	select PPC_BOOK3S_32
 	select PPC_FPU
 	select PPC_HAVE_PMU_SUPPORT
 	select PPC_HAVE_KUEP
 	select PPC_HAVE_KUAP
 
+config PPC_BOOK3S_601
+	bool "PowerPC 601"
+	select PPC_BOOK3S_32
+	select PPC_FPU
+	select PPC_HAVE_KUAP
+
 config PPC_85xx
 	bool "Freescale 85xx"
 	select E500