dm: doc: update of-plat with new phandle support

Update documentation to reflect the new phandle support when OF_PLATDATA
is used. Now phandles are implemented as pointers to U_BOOT_DEVICE,
which makes it possible to get a pointer to the actual device.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>