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/arch/powerpc/cpu/mpc8xx/immap.c b/arch/powerpc/cpu/mpc8xx/immap.c
index 2284979..dfe5dc2 100644
--- a/arch/powerpc/cpu/mpc8xx/immap.c
+++ b/arch/powerpc/cpu/mpc8xx/immap.c
@@ -12,7 +12,7 @@
 #include <common.h>
 #include <command.h>
 
-#include <asm/8xx_immap.h>
+#include <asm/immap_8xx.h>
 #include <commproc.h>
 #include <asm/iopin_8xx.h>
 #include <asm/io.h>