powerpc: MPC8536: Move CONFIG_MPC8536 to Kconfig option

Replace CONFIG_MPC8536 with ARCH_MPC8536 in Kconfig and clean up existing
macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/cpu/mpc85xx/Makefile b/arch/powerpc/cpu/mpc85xx/Makefile
index bfff58e..4733bfd 100644
--- a/arch/powerpc/cpu/mpc85xx/Makefile
+++ b/arch/powerpc/cpu/mpc85xx/Makefile
@@ -66,7 +66,7 @@
 
 # SoC specific SERDES support
 obj-$(CONFIG_ARCH_C29X)	+= c29x_serdes.o
-obj-$(CONFIG_MPC8536) += mpc8536_serdes.o
+obj-$(CONFIG_ARCH_MPC8536) += mpc8536_serdes.o
 obj-$(CONFIG_ARCH_MPC8544) += mpc8544_serdes.o
 obj-$(CONFIG_ARCH_MPC8548) += mpc8548_serdes.o
 obj-$(CONFIG_MPC8568) += mpc8568_serdes.o