dm: core: Call ofdata_to_platdata() with of-platdata

At present this function is never called when of-platdata is enabled since
we never have a device tree. However, this function is responsible for
copying over the of-platdata, so we must call it. Otherwise the probe()
method would have to be used.

Correct this and fix the sandbox serial driver to not read from the device
tree and try to write to what is read-only platdata on some platforms.

Fixes: 396e343b (dm: core: Allow binding a device from a live tree)
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>