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/speed.c b/arch/powerpc/cpu/mpc85xx/speed.c
index 38a28a8..2d3517b 100644
--- a/arch/powerpc/cpu/mpc85xx/speed.c
+++ b/arch/powerpc/cpu/mpc85xx/speed.c
@@ -202,7 +202,7 @@
 	}
 #endif
 
-#if defined(CONFIG_PPC_B4860) || defined(CONFIG_PPC_B4420) || \
+#if defined(CONFIG_ARCH_B4860) || defined(CONFIG_PPC_B4420) || \
 	defined(CONFIG_PPC_T2080) || defined(CONFIG_PPC_T2081)
 #define FM1_CLK_SEL	0xe0000000
 #define FM1_CLK_SHIFT	29