dtoc: Put phandle args in an array

We want to support more than one phandle argument. It makes sense to use
an array for this rather than discrete struct members. Adjust the code to
support this. Rename the member to 'arg' instead of 'id'.

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