powerpc: MPC8610: Remove macro CONFIG_MPC8610

Replace CONFIG_MPC8610 with ARCH_MPC8610 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 fe1859d..c3b3b07 100644
--- a/arch/powerpc/cpu/mpc86xx/Kconfig
+++ b/arch/powerpc/cpu/mpc86xx/Kconfig
@@ -13,6 +13,7 @@
 
 config TARGET_MPC8610HPCD
 	bool "Support MPC8610HPCD"
+	select ARCH_MPC8610
 
 config TARGET_MPC8641HPCN
 	bool "Support MPC8641HPCN"
@@ -22,6 +23,9 @@
 
 endchoice
 
+config ARCH_MPC8610
+	bool
+
 source "board/freescale/mpc8610hpcd/Kconfig"
 source "board/freescale/mpc8641hpcn/Kconfig"
 source "board/sbc8641d/Kconfig"