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/cpu/mpc86xx/Kconfig b/arch/powerpc/cpu/mpc86xx/Kconfig
index c3b3b07..98fb702 100644
--- a/arch/powerpc/cpu/mpc86xx/Kconfig
+++ b/arch/powerpc/cpu/mpc86xx/Kconfig
@@ -10,6 +10,7 @@
 
 config TARGET_SBC8641D
 	bool "Support sbc8641d"
+	select ARCH_MPC8641
 
 config TARGET_MPC8610HPCD
 	bool "Support MPC8610HPCD"
@@ -17,15 +18,20 @@
 
 config TARGET_MPC8641HPCN
 	bool "Support MPC8641HPCN"
+	select ARCH_MPC8641
 
 config TARGET_XPEDITE517X
 	bool "Support xpedite517x"
+	select ARCH_MPC8641
 
 endchoice
 
 config ARCH_MPC8610
 	bool
 
+config ARCH_MPC8641
+	bool
+
 source "board/freescale/mpc8610hpcd/Kconfig"
 source "board/freescale/mpc8641hpcn/Kconfig"
 source "board/sbc8641d/Kconfig"