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/spi/Kconfig b/drivers/spi/Kconfig
index 12dde8f..d3e407e 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -276,7 +276,7 @@
 
 config MPC8XX_SPI
 	bool "MPC8XX SPI Driver"
-	depends on 8xx
+	depends on MPC8xx
 	help
 	  Enable support for SPI on MPC8XX