keyboard: Add a migration message

A few boards still use the old keyboard mechanism. Set a deadline for them
to update to driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/driver-model/migration.rst b/doc/develop/driver-model/migration.rst
index 8bb8601..3dbeea6 100644
--- a/doc/develop/driver-model/migration.rst
+++ b/doc/develop/driver-model/migration.rst
@@ -98,3 +98,11 @@
 The I2C subsystem has supported the driver model since early 2015.
 Maintainers should submit patches switching over to using CONFIG_DM_I2C and
 other base driver model options in time for inclusion in the 2021.10 release.
+
+CONFIG_KEYBOARD
+---------------
+Deadline: 2022.10
+
+This is a legacy option which has been replaced by driver model.
+Maintainers should submit patches switching over to using CONFIG_DM_KEYBOARD and
+other base driver model options in time for inclusion in the 2022.10 release.