powerpc: T1040D4RDB: Separate from T1040RDB in Kconfig

Use TARGET_T1040D4RDB in Kconfig to simplify config macros. Replace
CONFIG_T1040D4RDB with TARGET_T1040D4RDB and clean up existing macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/freescale/t104xrdb/Kconfig b/board/freescale/t104xrdb/Kconfig
index 5b6095b..2a633c0 100644
--- a/board/freescale/t104xrdb/Kconfig
+++ b/board/freescale/t104xrdb/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_T1040RDB || TARGET_T1042RDB
+if TARGET_T1040RDB || TARGET_T1040D4RDB || TARGET_T1042RDB
 
 config SYS_BOARD
 	default "t104xrdb"