powerpc: Name xpn & x fields in HW Hash PTE format

Previously, the 36-bit code was using these bits, but they had
never been named in the pte format definition.  This patch just
gives those fields their proper names and adds a comment that
they are only present on some processors.

There is no functional code change.
Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/mmu-hash32.h

@@ -55,21 +55,25 @@ struct ppc_bat {
 
 #ifndef __ASSEMBLY__
 
-/* Hardware Page Table Entry */
+/*
+ * Hardware Page Table Entry
+ * Note that the xpn and x bitfields are used only by processors that
+ * support extended addressing; otherwise, those bits are reserved.
+ */
 struct hash_pte {
 	unsigned long v:1;	/* Entry is valid */
 	unsigned long vsid:24;	/* Virtual segment identifier */
 	unsigned long h:1;	/* Hash algorithm indicator */
 	unsigned long api:6;	/* Abbreviated page index */
 	unsigned long rpn:20;	/* Real (physical) page number */
-	unsigned long    :3;	/* Unused */
+	unsigned long xpn:3;	/* Real page number bits 0-2, optional */
 	unsigned long r:1;	/* Referenced */
 	unsigned long c:1;	/* Changed */
 	unsigned long w:1;	/* Write-thru cache mode */
 	unsigned long i:1;	/* Cache inhibited */
 	unsigned long m:1;	/* Memory coherence */
 	unsigned long g:1;	/* Guarded */
-	unsigned long  :1;	/* Unused */
+	unsigned long x:1;	/* Real page number bit 3, optional */
 	unsigned long pp:2;	/* Page protection */
 };