mx51evk: Add DM_SERIAL support

The conversion to DM_SERIAL is mandatory, so add support
for it.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/mx51evk_defconfig b/configs/mx51evk_defconfig
index b173648..832f718 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -57,6 +57,7 @@
 CONFIG_POWER_FSL=y
 CONFIG_POWER_SPI=y
 CONFIG_RTC_MC13XXX=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_MXC_SPI=y