hdl - Add de-interleave option for data reading

In the DDR memory, data are stored interleaved:

ch1[0], ch2[0], ch3[0], ch4[0], ch1[1], ch2[1], ch3[1], ch4[1]

This is because the DDR memory is used as a circular buffer (data are written straight to the DDR).

A option should be implemented to read the data in de-interleaved way:

ch1[0], ch1[1], ch2[0], ch2[1], ch3[0], ch3[1], ch4[0], ch4[1]

This should improve the overall readout performance, because without this option the re-ordering is done in software.