fdt/builder: enforce BE datatypes on add_prop_array

`add_prop_array` doesn't have any checks that the data is Big Endian. FDT requires that data is BE. Current usage kind of gets away with it, as we pass empty ranges or just u8 unformed entropy data. 

Perhaps we should use zerocopy BE byte types for `add_prop_array` and also perhaps `prop_array_iter`? 