powerpc: Fix a build problem on ppc32 with new DMA_ATTRs

The new dma_attrs support must only be enabled for 64 bits as it's not
been implemented for 32 bits yet.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/Kconfig

@@ -113,7 +113,7 @@ config PPC
 	select HAVE_KPROBES
 	select HAVE_KRETPROBES
 	select HAVE_LMB
-	select HAVE_DMA_ATTRS
+	select HAVE_DMA_ATTRS if PPC64
 
 config EARLY_PRINTK
 	bool