powerpc: P1020RDB-PC: Separate from P1_P2_RDB_PC in Kconfig

Use TARGET_P1020RDB_PC instead of sharing with TARGET_P1_P2_RDB_PC
to simplify Kconfig and config macros.

Remove macro CONFIG_P1020RDB_PC.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/cpu/mpc85xx/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index 636e26f..d7b3763 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -120,6 +120,11 @@
 	bool "Support P1020MBG-PC"
 	select SUPPORT_SPL
 	select SUPPORT_TPL
+config TARGET_P1020RDB_PC
+	bool "Support P1020RDB-PC"
+	select SUPPORT_SPL
+	select SUPPORT_TPL
+
 
 config TARGET_P1_P2_RDB_PC
 	bool "Support p1_p2_rdb_pc"