powerpc: mpc8540: Remove macro CONFIG_MPC8540

Replace CONFIG_MPC8540 with ARCH_MPC8540 in Kconfig and clean up existing
macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/include/asm/config_mpc85xx.h b/arch/powerpc/include/asm/config_mpc85xx.h
index be1ab4a..7f9b2d9 100644
--- a/arch/powerpc/include/asm/config_mpc85xx.h
+++ b/arch/powerpc/include/asm/config_mpc85xx.h
@@ -44,7 +44,7 @@
 #define CONFIG_SYS_FSL_ERRATUM_A004508
 #define CONFIG_SYS_FSL_ERRATUM_A005125
 
-#elif defined(CONFIG_MPC8540)
+#elif defined(CONFIG_ARCH_MPC8540)
 #define CONFIG_MAX_CPUS			1
 #define CONFIG_SYS_FSL_NUM_LAWS		8
 #define CONFIG_SYS_FSL_DDRC_GEN1