mtd: replace CONFIG_8xx by CONFIG_PPC_8xx

Two config options exist to define powerpc MPC8xx:
* CONFIG_PPC_8xx
* CONFIG_8xx
In addition, CONFIG_PPC_8xx also defines CONFIG_CPM1 as
communication co-processor

arch/powerpc/platforms/Kconfig.cputype has contained the following
comment about CONFIG_8xx item for some years:
"# this is temp to handle compat with arch=ppc"

It looks like not many places still have that old CONFIG_8xx used,
so it is likely to be a good time to get rid of it completely ?
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/maps/Kconfig

@@ -242,7 +242,7 @@ config MTD_L440GX
 
 config MTD_CFI_FLAGADM
 	tristate "CFI Flash device mapping on FlagaDM"
-	depends on 8xx && MTD_CFI
+	depends on PPC_8xx && MTD_CFI
 	help
 	  Mapping for the Flaga digital module. If you don't have one, ignore
 	  this setting.