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/board/freescale/p1_p2_rdb_pc/ddr.c b/board/freescale/p1_p2_rdb_pc/ddr.c
index 1f3793b..a121256 100644
--- a/board/freescale/p1_p2_rdb_pc/ddr.c
+++ b/board/freescale/p1_p2_rdb_pc/ddr.c
@@ -78,7 +78,7 @@
 	.refresh_rate_ps = 7800000,
 	.tfaw_ps = 30000,
 };
-#elif (defined(CONFIG_P1020MBG) || defined(CONFIG_P1020RDB_PD))
+#elif (defined(CONFIG_TARGET_P1020MBG) || defined(CONFIG_P1020RDB_PD))
 /* Micron MT41J512M8_187E */
 dimm_params_t ddr_raw_timing = {
 	.n_ranks = 2,