dm: core: Add a function to see if a device exists

All the uclass functions for finding a device end up creating a uclass
if it doesn't exist. Add a function which instead returns NULL in this
case.

This is useful when in the 'unbind' path, since we don't want to undo
any unbinding which has already happened.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/dm/uclass.h

@@ -435,6 +435,17 @@ int uclass_next_device_check(struct udevice **devp);
 int uclass_first_device_drvdata(enum uclass_id id, ulong driver_data,
 				struct udevice **devp);
 
+/**
+ * uclass_try_first_device()- See if there is a device for a uclass
+ *
+ * If the uclass exists, this returns the first device on that uclass, without
+ * probing it. If the uclass does not exist, it gives up
+ *
+ * @id: Uclass ID to check
+ * Return: Pointer to device, if found, else NULL
+ */
+struct udevice *uclass_try_first_device(enum uclass_id id);
+
 /**
  * uclass_probe_all() - Probe all devices based on an uclass ID
  *