dm: core: precise comments for get/find device by name

The functions:
- uclass_find_device_by_name()
- uclass_get_device_by_name()
searches the required device for the exactly given name.
This patch, presice this fact for both function's comments.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>