dm: core: device: add function: dev_get_uclass_name()

This commit extends the driver model device's API by function:
- dev_get_uclass_name()

And this function returns the device's uclass driver name if:
- given dev pointer, is non_NULL
otherwise, the NULL pointer is returned.

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