wandboard: Select DM_SERIAL

The conversion to DM_SERIAL is mandatory, so select this
option.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/wandboard_defconfig b/configs/wandboard_defconfig
index 70b4cf9..33d9db5 100644
--- a/configs/wandboard_defconfig
+++ b/configs/wandboard_defconfig
@@ -72,6 +72,7 @@
 CONFIG_DM_PMIC=y
 CONFIG_DM_PMIC_PFUZE100=y
 CONFIG_DM_SCSI=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_DM_THERMAL=y
 CONFIG_USB=y