board: sl28: enable DM_SERIAL

With all preparations in place, switch over to DM_SERIAL.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index fa98c1a..b39b043 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1616,6 +1616,7 @@
 	select DM_RNG
 	select DM_RTC
 	select DM_SCSI
+	select DM_SERIAL
 	select DM_SPI
 	select DM_USB
 	select SPL_DM if SPL