powerpc: set CONFIG_PPC_OF=y always for ARCH=powerpc

The CONFIG_PPC_OF symbol is used to mean that the firmware device tree
access functions are available.  Since we always have a device tree
with ARCH=powerpc, make CONFIG_PPC_OF always Y for ARCH=powerpc.

This fixes some compile errors reported by Kumar Gala, but in a
different way to his patch.  This also makes prom_parse.o be compiled
only if CONFIG_PPC_OF so that non-OF ARCH=ppc platforms will compile.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 5692edb..427ae0f 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -329,9 +329,7 @@
 	select MMIO_NVRAM
 
 config PPC_OF
-	bool
-	depends on PPC_MULTIPLATFORM	# for now
-	default y
+	def_bool y
 
 config XICS
 	depends on PPC_PSERIES
@@ -399,7 +397,6 @@
 config IBMEBUS
 	depends on PPC_PSERIES
 	bool "Support for GX bus based adapters"
-	default y
 	help
 	  Bus device driver for GX bus based adapters.