powerpc: ARCH_PFN_OFFSET should be unsigned long

pfns are unsigned long, but MEMORY_START is phys_addr_t.  This leads
to page_to_pfn() returning phys_addr_t, and thus type mismatches in a few
print statements.
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/page.h

@@ -100,7 +100,7 @@ extern phys_addr_t kernstart_addr;
 #endif
 
 #ifdef CONFIG_FLATMEM
-#define ARCH_PFN_OFFSET		(MEMORY_START >> PAGE_SHIFT)
+#define ARCH_PFN_OFFSET		((unsigned long)(MEMORY_START >> PAGE_SHIFT))
 #define pfn_valid(pfn)		((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
 #endif