powerpc: mpc8541: Remove macro CONFIG_MPC8541

Replace CONFIG_MPC8541 with ARCH_MPC8541 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 69567db..8b08ee2 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -64,6 +64,7 @@
 
 config TARGET_MPC8541CDS
 	bool "Support MPC8541CDS"
+	select ARCH_MPC8541
 
 config TARGET_MPC8544DS
 	bool "Support MPC8544DS"
@@ -197,6 +198,9 @@
 config ARCH_MPC8540
 	bool
 
+config ARCH_MPC8541
+	bool
+
 config ARCH_MPC8544
 	bool