powerpc: P1020MBG: Separate from P1_P2_RDB_PC in Kconfig

Use TARGET_P1020MBG instead of sharing with TARGET_P1_P2_RDB_PC to
simplify Kconfig and other macros.

Remove macro CONFIG_P1020MBG.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/cpu/mpc85xx/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index d0c57a1..636e26f 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -116,6 +116,11 @@
 	bool "Support P1023RDB"
 	select ARCH_P1023
 
+config TARGET_P1020MBG
+	bool "Support P1020MBG-PC"
+	select SUPPORT_SPL
+	select SUPPORT_TPL
+
 config TARGET_P1_P2_RDB_PC
 	bool "Support p1_p2_rdb_pc"
 	select SUPPORT_SPL