dm: core: introduce uclass_get_device_by_of_path()

There's quite a few instances of board-specific code doing

  off = fdt_path_offset(gd->fdt_blob, ...);
  ...
  ret = uclass_get_device_by_of_offset(..., off, &dev);

looking for an eeprom or a pmic via some alias. Such code can be
simplified a little if we have a helper for directly getting a device
via device tree path (including being given as an alias).

Implement it in terms of ofnode rather than raw offsets so that this
will work whether live tree is enabled or not.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>