powerpc: P1021: Remove macro CONFIG_P1021

Replace CONFIG_P1021 with ARCH_P1021 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 9142b33..5550687 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -144,6 +144,7 @@
 	bool "Support P1021RDB"
 	select SUPPORT_SPL
 	select SUPPORT_TPL
+	select ARCH_P1021
 
 config TARGET_P1024RDB
 	bool "Support P1024RDB"
@@ -287,6 +288,9 @@
 config ARCH_P1020
 	bool
 
+config ARCH_P1021
+	bool
+
 config ARCH_P1022
 	bool