fdt: Add function to alloc phandle values

If we are creating reference (handles) to nodes in a device tree we need
to first create a new phandle in node and this needs a new phandle
value.  So we search through the whole dtb to find the max phandle value
and return the next greater value for a new phandle allocation.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>