powerpc: Fix invalid construct in our CPU selection Kconfig

commit 5b7c3c91 introduced an invalid
construct in our CPU selection. This caused warnings, though it still
appeared to do the right thing.

This fixes it properly by having separate formal definitions of
PPC_BOOK3S_32 and PPC_BOOK3S_64 and one statement defining
PPC_BOOK3S based on the two above.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/Kconfig.cputype

@@ -21,7 +21,7 @@ choice
 
 	  If unsure, select 52xx/6xx/7xx/74xx/82xx/83xx/86xx.
 
-config PPC_BOOK3S
+config PPC_BOOK3S_32
 	bool "512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx"
 	select PPC_FPU
 
@@ -57,11 +57,14 @@ config E200
 
 endchoice
 
-config PPC_BOOK3S
-	default y
+config PPC_BOOK3S_64
+	def_bool y
 	depends on PPC64
 	select PPC_FPU
 
+config PPC_BOOK3S
+	def_bool y
+	depends on PPC_BOOK3S_32 || PPC_BOOK3S_64
 
 config POWER4_ONLY
 	bool "Optimize for POWER4"