dtoc: Handle nodes with phandles that depend on the same

At present dtoc assumes that nodes which are phandles do not themselves
reference other phandle nodes. Unfortunately this is not necessarilly
true. As a result we can currently output C code which does not compile
because a node declaration can be referenced before it is declared.

Adjust the code to explicitly output all phandle nodes needed by node
before the node itself is output.

This fixes building with the latest rk3399-firefly.dts from Linux, which
has reordered the nodes.

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