clean up dma code (#1094)
clean up dma code (#1094)

* clean up dma code

* replace xlnk with allocate() call

* fix dma alignment bug and add more docs
yunqu/PYNQ,Xilinx/PYNQ,cathalmccabe/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,yunqu/PYNQ,cathalmccabe/PYNQ,schelleg/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,yunqu/PYNQ,Xilinx/PYNQ,Xilinx/PYNQ,schelleg/PYNQ,yunqu/PYNQ,Xilinx/PYNQ,cathalmccabe/PYNQ,Xilinx/PYNQ,cathalmccabe/PYNQ,schelleg/PYNQ,yunqu/PYNQ