powerpc: B4420: Remove macro CONFIG_PPC_B4420

Replace CONFIG_PPC_B4420 with ARCH_B4420 in Kconfig and clean up
existing macros.
diff --git a/arch/powerpc/cpu/mpc85xx/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index 962ec2b..acac21e 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -18,6 +18,7 @@
 
 config TARGET_B4420QDS
 	bool "Support B4420QDS"
+	select ARCH_B4420
 	select SUPPORT_SPL
 	select PHYS_64BIT
 
@@ -257,6 +258,9 @@
 
 endchoice
 
+config ARCH_B4420
+	bool
+
 config ARCH_B4860
 	bool