arm: s5p4418: dm_serial: switch to DM_SERIAL

Switch the S5P4418-SOC and therefore the s5p4418_nanopi2 board to
DM_SERIAL.

Signed-off-by: Stefan Bosch <stefan_b@posteo.net>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 37d3f82..01fef3f 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -59,6 +59,7 @@
 obj-$(CONFIG_HTIF_CONSOLE) += serial_htif.o
 obj-$(CONFIG_SIFIVE_SERIAL) += serial_sifive.o
 obj-$(CONFIG_XEN_SERIAL) += serial_xen.o
+obj-$(CONFIG_S5P4418_PL011_SERIAL) += serial_s5p4418_pl011.o
 
 ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_USB_TTY) += usbtty.o