powerpc: turn CONFIG_OF into a select

so that we can make CONFIG_OF global and remove it from
the architecture Kconfig files later.
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

arch/powerpc/Kconfig

@@ -120,6 +120,8 @@ config ARCH_NO_VIRT_TO_BUS
 config PPC
 	bool
 	default y
+	select OF
+	select OF_FLATTREE
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_FUNCTION_TRACER
@@ -173,8 +175,7 @@ config PPC_OF
 	def_bool y
 
 config OF
-	def_bool y
-	select OF_FLATTREE
+	bool
 
 config PPC_UDBG_16550
 	bool