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/drivers/net/Kconfig b/drivers/net/Kconfig
index f589978..608ed71 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -331,7 +331,7 @@
 
 config MPC8XX_FEC
 	bool "Fast Ethernet Controller on MPC8XX"
-	depends on 8xx
+	depends on MPC8xx
 	select MII
 	help
 	  This driver implements support for the Fast Ethernet Controller