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/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index 90777db..51192fe 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -162,6 +162,7 @@
 	bool "Support P2020RDB-PC"
 	select SUPPORT_SPL
 	select SUPPORT_TPL
+	select ARCH_P2020
 
 config TARGET_P1_TWR
 	bool "Support p1_twr"
@@ -232,6 +233,7 @@
 
 config TARGET_XPEDITE550X
 	bool "Support xpedite550x"
+	select ARCH_P2020
 
 config TARGET_UCP1020
 	bool "Support uCP1020"
@@ -306,6 +308,9 @@
 config ARCH_P1025
 	bool
 
+config ARCH_P2020
+	bool
+
 source "board/freescale/b4860qds/Kconfig"
 source "board/freescale/bsc9131rdb/Kconfig"
 source "board/freescale/bsc9132qds/Kconfig"