ruby: dma sequencer: remove RubyPort as parent class

ruby: dma sequencer: remove RubyPort as parent class
As of now DMASequencer inherits from the RubyPort class.  But the code in
RubyPort class is heavily tailored for the CPU Sequencer.  There are parts of
the code that are not required at all for the DMA sequencer.  Moreover, the
next patch uses the dma sequencer for carrying out memory accesses for all the
io devices.  Hence, it is better to have a leaner dma sequencer.