powerpc: P2020: Remove macro CONFIG_P2020

Replace CONFIG_P2020 with ARCH_P2020 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 a3fba03..acf57b7 100644
--- a/arch/powerpc/cpu/mpc85xx/Makefile
+++ b/arch/powerpc/cpu/mpc85xx/Makefile
@@ -81,7 +81,7 @@
 obj-$(CONFIG_ARCH_P1024)	+= p1021_serdes.o
 obj-$(CONFIG_ARCH_P1025)	+= p1021_serdes.o
 obj-$(CONFIG_P2010)	+= p2020_serdes.o
-obj-$(CONFIG_P2020)	+= p2020_serdes.o
+obj-$(CONFIG_ARCH_P2020)	+= p2020_serdes.o
 obj-$(CONFIG_PPC_P2041) += p2041_serdes.o
 obj-$(CONFIG_PPC_P3041) += p3041_serdes.o
 obj-$(CONFIG_PPC_P4080) += p4080_serdes.o