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/Makefile b/arch/powerpc/cpu/mpc86xx/Makefile
index 0caf9f1..88c23fc 100644
--- a/arch/powerpc/cpu/mpc86xx/Makefile
+++ b/arch/powerpc/cpu/mpc86xx/Makefile
@@ -20,5 +20,5 @@
 obj-y	+= interrupts.o
 obj-$(CONFIG_MP) += mp.o
 obj-$(CONFIG_ARCH_MPC8610) += mpc8610_serdes.o
-obj-$(CONFIG_MPC8641) += mpc8641_serdes.o
+obj-$(CONFIG_ARCH_MPC8641) += mpc8641_serdes.o
 obj-y	+= speed.o