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/cpu.c b/arch/powerpc/cpu/mpc86xx/cpu.c
index 4dc483b..7a9570c 100644
--- a/arch/powerpc/cpu/mpc86xx/cpu.c
+++ b/arch/powerpc/cpu/mpc86xx/cpu.c
@@ -92,7 +92,7 @@
 	if (get_l2cr() & 0x80000000) {
 #if defined(CONFIG_ARCH_MPC8610)
 		puts("256");
-#elif defined(CONFIG_MPC8641)
+#elif defined(CONFIG_ARCH_MPC8641)
 		puts("512");
 #endif
 		puts(" KiB enabled\n");