powerpc/t208xrdb: Update MAINTAINERS file

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Shengzhou Liu <Shengzhou.Liu@nxp.com>

board/freescale/t208xrdb/MAINTAINERS

 T208XRDB BOARD
-#M:	-
+M:	Shengzhou Liu <Shengzhou.Liu@nxp.com>
 S:	Maintained
 F:	board/freescale/t208xrdb/
 F:	include/configs/T208xRDB.h