powerpc/8xx: remove CONFIG_8xx

Two config options exist to define powerpc MPC8xx:
* CONFIG_PPC_8xx
* CONFIG_8xx

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"

There is no more users of CONFIG_8xx, so remove it.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/Kconfig.cputype

@@ -32,7 +32,6 @@ config PPC_85xx
 config PPC_8xx
 	bool "Freescale 8xx"
 	select FSL_SOC
-	select 8xx
 	select PPC_LIB_RHEAP
 	select SYS_SUPPORTS_HUGETLBFS
 
@@ -149,10 +148,6 @@ config 6xx
 	depends on PPC32 && PPC_BOOK3S
 	select PPC_HAVE_PMU_SUPPORT
 
-# this is temp to handle compat with arch=ppc
-config 8xx
-	bool
-
 config E500
 	select FSL_EMB_PERFMON
 	select PPC_FSL_BOOK3E