powerpc: P1020: Remove macro CONFIG_P1020

Replace CONFIG_P1020 with ARCH_P1020 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 49dcbe3..26a6786 100644
--- a/arch/powerpc/cpu/mpc85xx/Makefile
+++ b/arch/powerpc/cpu/mpc85xx/Makefile
@@ -74,7 +74,7 @@
 obj-$(CONFIG_ARCH_MPC8572) += mpc8572_serdes.o
 obj-$(CONFIG_ARCH_P1010)	+= p1010_serdes.o
 obj-$(CONFIG_ARCH_P1011)	+= p1021_serdes.o
-obj-$(CONFIG_P1020)	+= p1021_serdes.o
+obj-$(CONFIG_ARCH_P1020)	+= p1021_serdes.o
 obj-$(CONFIG_P1021)	+= p1021_serdes.o
 obj-$(CONFIG_ARCH_P1022)	+= p1022_serdes.o
 obj-$(CONFIG_ARCH_P1023)	+= p1023_serdes.o