[POWERPC] Allow combined iSeries and MULTIPLATFORM build

This will build with ISERIES, PSERIES and PMAC64 selected, but will only
boot on iSeries so far.
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>

arch/powerpc/Kconfig

@@ -338,10 +338,6 @@ config PPC_MULTIPLATFORM
 	  RS/6000 machine, an Apple machine, or a PReP, CHRP,
 	  Maple or Cell-based machine.
 
-config PPC_ISERIES
-	bool "IBM Legacy iSeries"
-	depends on PPC64
-
 config EMBEDDED6xx
 	bool "Embedded 6xx/7xx/7xxx-based board"
 	depends on PPC32 && (BROKEN||BROKEN_ON_SMP)
@@ -365,6 +361,10 @@ config PPC_PSERIES
 	select PPC_UDBG_16550
 	default y
 
+config PPC_ISERIES
+	bool "IBM Legacy iSeries"
+	depends on PPC_MULTIPLATFORM && PPC64
+
 config PPC_CHRP
 	bool "Common Hardware Reference Platform (CHRP) based machines"
 	depends on PPC_MULTIPLATFORM && PPC32