powerpc: MPC8560: Remove macro CONFIG_MPC8560

Replace CONFIG_MPC8560 with ARCH_MPC8560 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 c58f76f..98f98af 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -80,6 +80,7 @@
 
 config TARGET_MPC8560ADS
 	bool "Support MPC8560ADS"
+	select ARCH_MPC8560
 
 config TARGET_MPC8568MDS
 	bool "Support MPC8568MDS"
@@ -211,6 +212,9 @@
 config ARCH_MPC8555
 	bool
 
+config ARCH_MPC8560
+	bool
+
 source "board/freescale/b4860qds/Kconfig"
 source "board/freescale/bsc9131rdb/Kconfig"
 source "board/freescale/bsc9132qds/Kconfig"