dm: core: Add a way of overriding the ACPI device path

Some devices such as GPIO need to override the normal path that would be
generated by driver model. Add a device-tree property for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>