powerpc: mpc8xx: Change CONFIG_8xx to CONFIG_MPC8xx

CONFIG_8xx doesn't mean much outside of arch/powerpc/
This patch renames it CONFIG_MPC8xx just like CONFIG_MPC85xx etc ...
It also renames 8xx_immap.h to immap_8xx.h to be consistent with
other file names.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
diff --git a/include/watchdog.h b/include/watchdog.h
index 64b59f1..d420ab1 100644
--- a/include/watchdog.h
+++ b/include/watchdog.h
@@ -73,7 +73,7 @@
  */
 
 /* MPC 8xx */
-#if defined(CONFIG_8xx) && !defined(__ASSEMBLY__)
+#if defined(CONFIG_MPC8xx) && !defined(__ASSEMBLY__)
 	void reset_8xx_watchdog(immap_t __iomem *immr);
 #endif