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/arch/powerpc/cpu/mpc85xx/b4860_serdes.c b/arch/powerpc/cpu/mpc85xx/b4860_serdes.c
index 63172de..64db7a7 100644
--- a/arch/powerpc/cpu/mpc85xx/b4860_serdes.c
+++ b/arch/powerpc/cpu/mpc85xx/b4860_serdes.c
@@ -15,7 +15,7 @@
 	u8 lanes[SRDS_MAX_LANES];
 };
 
-#ifdef CONFIG_PPC_B4860
+#ifdef CONFIG_ARCH_B4860
 static struct serdes_config serdes1_cfg_tbl[] = {
 	/* SerDes 1 */
 	{0x01, {AURORA, AURORA, CPRI6, CPRI5,