powerpc: B4420: Remove macro CONFIG_PPC_B4420

Replace CONFIG_PPC_B4420 with ARCH_B4420 in Kconfig and clean up
existing macros.
diff --git a/drivers/net/fm/Makefile b/drivers/net/fm/Makefile
index e136f7c..73bd31d 100644
--- a/drivers/net/fm/Makefile
+++ b/drivers/net/fm/Makefile
@@ -35,7 +35,7 @@
 obj-$(CONFIG_PPC_T4240) += t4240.o
 obj-$(CONFIG_PPC_T4160) += t4240.o
 obj-$(CONFIG_PPC_T4080) += t4240.o
-obj-$(CONFIG_PPC_B4420) += b4860.o
+obj-$(CONFIG_ARCH_B4420) += b4860.o
 obj-$(CONFIG_ARCH_B4860) += b4860.o
 obj-$(CONFIG_LS1043A)	+= ls1043.o
 obj-$(CONFIG_ARCH_LS1046A)	+= ls1046.o