dm: core: Drop uclass_find_device_by_phandle() with of-platdata

At present this function is included in the build but with of-platdata it
only services to produce a confusing link error complaining about a call
to dev_read_u32_default().

Drop it so that any call to uclass_find_device_by_phandle() is flagged as
an error, making it easier to see what is going on.

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