powerpc: Allow platforms to force-enable CONFIG_SMP

Of the 64-bit Book3S platforms, only powermac supports booting on an
actual non-SMP system. The other platforms can be built with SMP
disabled, but it doesn't make a lot of sense given the CPUs they support
are all multicore or multithreaded.

So give platforms the option of forcing SMP=y.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/Kconfig.cputype

@@ -371,10 +371,16 @@ config PPC_PERF_CTRS
        help
          This enables the powerpc-specific perf_event back-end.
 
+config FORCE_SMP
+	# Allow platforms to force SMP=y by selecting this
+	bool
+	default n
+	select SMP
+
 config SMP
 	depends on PPC_BOOK3S || PPC_BOOK3E || FSL_BOOKE || PPC_47x
 	select GENERIC_IRQ_MIGRATION
-	bool "Symmetric multi-processing support"
+	bool "Symmetric multi-processing support" if !FORCE_SMP
 	---help---
 	  This enables support for systems with more than one CPU. If you have
 	  a system with only one CPU, say N. If you have a system with more