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/speed.c b/arch/powerpc/cpu/mpc85xx/speed.c
index 2d3517b..a392414 100644
--- a/arch/powerpc/cpu/mpc85xx/speed.c
+++ b/arch/powerpc/cpu/mpc85xx/speed.c
@@ -202,7 +202,7 @@
 	}
 #endif
 
-#if defined(CONFIG_ARCH_B4860) || defined(CONFIG_PPC_B4420) || \
+#if defined(CONFIG_ARCH_B4860) || defined(CONFIG_ARCH_B4420) || \
 	defined(CONFIG_PPC_T2080) || defined(CONFIG_PPC_T2081)
 #define FM1_CLK_SEL	0xe0000000
 #define FM1_CLK_SHIFT	29