dm: core: uclass: add function: uclass_get_device_by_name()

This commit extends the driver model uclass's API by function:
- uclass_get_device_by_name()

And this function returns the device if:
- uclass with given ID, exists,
- device with exactly given name(dev->name), exists,
- device probe, doesn't return an error.

The returned device is activated and ready to use.

Note:
This function returns the first device, which name is equal
to the given one. This means, that using this function you must
assume, that the device name is unique in the given uclass's ID
device list.

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