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/cpu/mpc85xx/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index 682d4e7..69567db 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -60,6 +60,7 @@
 
 config TARGET_MPC8540ADS
 	bool "Support MPC8540ADS"
+	select ARCH_MPC8540
 
 config TARGET_MPC8541CDS
 	bool "Support MPC8541CDS"
@@ -193,6 +194,9 @@
 config ARCH_MPC8536
 	bool
 
+config ARCH_MPC8540
+	bool
+
 config ARCH_MPC8544
 	bool