powerpc: B4860: Remove macro CONFIG_PPC_B4860

Replace CONFIG_PPC_B4860 with ARCH_B4860 in Kconfig and clean up
existing macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/drivers/net/fm/Makefile b/drivers/net/fm/Makefile
index d9bcec5..e136f7c 100644
--- a/drivers/net/fm/Makefile
+++ b/drivers/net/fm/Makefile
@@ -36,6 +36,6 @@
 obj-$(CONFIG_PPC_T4160) += t4240.o
 obj-$(CONFIG_PPC_T4080) += t4240.o
 obj-$(CONFIG_PPC_B4420) += b4860.o
-obj-$(CONFIG_PPC_B4860) += b4860.o
+obj-$(CONFIG_ARCH_B4860) += b4860.o
 obj-$(CONFIG_LS1043A)	+= ls1043.o
 obj-$(CONFIG_ARCH_LS1046A)	+= ls1046.o