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/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index f755bc8..9142b33 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -120,20 +120,25 @@
 	bool "Support P1020MBG-PC"
 	select SUPPORT_SPL
 	select SUPPORT_TPL
+	select ARCH_P1020
+
 config TARGET_P1020RDB_PC
 	bool "Support P1020RDB-PC"
 	select SUPPORT_SPL
 	select SUPPORT_TPL
+	select ARCH_P1020
 
 config TARGET_P1020RDB_PD
 	bool "Support P1020RDB-PD"
 	select SUPPORT_SPL
 	select SUPPORT_TPL
+	select ARCH_P1020
 
 config TARGET_P1020UTM
 	bool "Support P1020UTM"
 	select SUPPORT_SPL
 	select SUPPORT_TPL
+	select ARCH_P1020
 
 config TARGET_P1021RDB
 	bool "Support P1021RDB"
@@ -226,6 +231,7 @@
 
 config TARGET_UCP1020
 	bool "Support uCP1020"
+	select ARCH_P1020
 
 config TARGET_CYRUS
 	bool "Support Varisys Cyrus"
@@ -278,6 +284,9 @@
 config ARCH_P1011
 	bool
 
+config ARCH_P1020
+	bool
+
 config ARCH_P1022
 	bool