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/cpu.c b/arch/powerpc/cpu/mpc85xx/cpu.c
index 3e6f8f3..268429b 100644
--- a/arch/powerpc/cpu/mpc85xx/cpu.c
+++ b/arch/powerpc/cpu/mpc85xx/cpu.c
@@ -294,7 +294,7 @@
 {
 /* Everything after the first generation of PQ3 parts has RSTCR */
 #if defined(CONFIG_ARCH_MPC8540) || defined(CONFIG_ARCH_MPC8541) || \
-	defined(CONFIG_ARCH_MPC8555) || defined(CONFIG_MPC8560)
+	defined(CONFIG_ARCH_MPC8555) || defined(CONFIG_ARCH_MPC8560)
 	unsigned long val, msr;
 
 	/*