defconfig: dra74_evm: enable serial driver model

Enable serial driver model for dra74_evm as ns16550 supports
driver model.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

configs/dra74_evm_defconfig

@@ -16,5 +16,6 @@ CONFIG_DM=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_BAR=y
 CONFIG_SPI_FLASH_SPANSION=y
+CONFIG_DM_SERIAL=y
 CONFIG_SYS_NS16550=y
 CONFIG_TI_QSPI=y