serdev: Do not return -ENODEV from of_serdev_register_devices() if external connector is used

If an external connector like M.2 is connected to the serdev controller
in DT, then the serdev devices may be created dynamically by the connector
driver. So do not return -ENODEV from of_serdev_register_devices() if the
static nodes are not found and the graph node is used.

Tested-by: Hans de Goede <johannes.goede@oss.qualcomm.com> # ThinkPad T14s gen6 (arm64)
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260326-pci-m2-e-v7-3-43324a7866e6@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

Author: Mani-Sadhasivam

drivers/tty/serdev/core.c

@@ -12,6 +12,7 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/of.h>
+#include <linux/of_graph.h>
 #include <linux/of_device.h>
 #include <linux/pm_domain.h>
 #include <linux/pm_runtime.h>
@@ -561,7 +562,13 @@ static int of_serdev_register_devices(struct serdev_controller *ctrl)
 		} else
 			found = true;
 	}
-	if (!found)
+
+	/*
+	 * When the serdev controller is connected to an external connector like
+	 * M.2 in DT, then the serdev devices may be created dynamically by the
+	 * connector driver.
+	 */
+	if (!found && !of_graph_is_present(dev_of_node(&ctrl->dev)))
 		return -ENODEV;
 
 	return 0;