xen/mmu: Correct PAT MST setting.

Jan Beulich spotted that the PAT MSR settings in the Xen public
document that "the first (PAT6) column was wrong across the
board, and the column for PAT7 was missing altogether."

This updates it to be in sync.

CC: Jan Beulich <jbeulich@suse.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: NJan Beulich <jbeulich@suse.com>

arch/x86/xen/mmu.c

@@ -468,8 +468,8 @@ PV_CALLEE_SAVE_REGS_THUNK(xen_pgd_val);
  * 3        PCD PWT      UC       UC     UC
  * 4    PAT              WB       WC     WB
  * 5    PAT     PWT      WC       WP     WT
- * 6    PAT PCD          UC-      UC     UC-
- * 7    PAT PCD PWT      UC       UC     UC
+ * 6    PAT PCD          UC-      rsv    UC-
+ * 7    PAT PCD PWT      UC       rsv    UC
  */
 
 void xen_set_pat(u64 pat)