powerpc: MPC8641: Remove macro CONFIG_MPC8641

Replace CONFIG_MPC8641 with ARCH_MPC8641 in Kconfig and clean up
existing macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/include/asm/config_mpc86xx.h b/arch/powerpc/include/asm/config_mpc86xx.h
index 96340af..e3d94e3 100644
--- a/arch/powerpc/include/asm/config_mpc86xx.h
+++ b/arch/powerpc/include/asm/config_mpc86xx.h
@@ -15,7 +15,7 @@
 #define CONFIG_MAX_CPUS			1
 #define CONFIG_SYS_FSL_NUM_LAWS		10
 
-#elif defined(CONFIG_MPC8641)
+#elif defined(CONFIG_ARCH_MPC8641)
 #define CONFIG_MAX_CPUS			2
 #define CONFIG_SYS_FSL_NUM_LAWS		10