dm: serial: Add a migration deadline for serial

This probably should have been done a while back since it is a core
system. Very few boards remain to be migrated.

Addd a migration deadline for a year out.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 10879f1..2121245 100644
--- a/Makefile
+++ b/Makefile
@@ -1133,6 +1133,7 @@
 	@# confuses this rule. Use if() to send just a single character which
 	@# is enable to tell 'deprecated' that one of these symbols exists
 	$(call deprecated,CONFIG_TIMER,Timer drivers,v2023.01,$(if $(strip $(CONFIG_SYS_TIMER_RATE)$(CONFIG_SYS_TIMER_COUNTER)),x))
+	$(call deprecated,CONFIG_DM_SERIAL,Serial drivers,v2023.04,$(CONFIG_SERIAL))
 	@# Check that this build does not use CONFIG options that we do not
 	@# know about unless they are in Kconfig. All the existing CONFIG
 	@# options are whitelisted, so new ones should not be added.