imx: mx7dsabresd: enable DM_SERIAL

Enable CONFIG_DM_SERIAL, and `dm tree` could show:
 serial        1  [   ]   serial_mxc            |       |-- serial@30a80000

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/mx7dsabresd_defconfig b/configs/mx7dsabresd_defconfig
index 28fc367..d8c5dbc 100644
--- a/configs/mx7dsabresd_defconfig
+++ b/configs/mx7dsabresd_defconfig
@@ -67,6 +67,7 @@
 CONFIG_DM_REGULATOR_PFUZE100=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y