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/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
index bf37ef2..03b25f9 100644
--- a/arch/powerpc/kernel/head_32.S
+++ b/arch/powerpc/kernel/head_32.S
@@ -120,10 +120,12 @@
  * because OF may have I/O devices mapped into that area
  * (particularly on CHRP).
  */
+#ifdef CONFIG_PPC_MULTIPLATFORM
 	cmpwi	0,r5,0
 	beq	1f
 	bl	prom_init
 	trap
+#endif
 
 /*
  * Check for BootX signature when supporting PowerMac and branch to