dm: core: Read parent ofdata before children

At present a device can read its ofdata before its parent has done the
same. This can cause problems in the case where the parent has a 'ranges'
property, thus affecting the operation of dev_read_addr(), for example.

We already probe parent devices before children so it does not seem to be
a large step to do the same with ofdata.

Make the change and update the documentation in this area.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Ley Foon Tan <ley.foon.tan@intel.com>