dtoc: Put each phandle on a separate line

When writing values from properties which contain phandles, dtoc currently
writes 8 phandles per line. Change this to write one phandle per line.
This helps reduce line length, since phandles are generally longer and may
have arguments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>