ruby: make DMASequencer inherit from RubyPort

ruby: make DMASequencer inherit from RubyPort

This patch essentially rolls back 10518:30e3715c9405 to make RubyPort the
parent class of DMASequencer.  It removes redundant code and restores some
features which were lost when directly inheriting from MemObject.  For
example,
DMASequencer can now communicate to other devices using PIO, which is useful
for memmory-mapped communication between multiple DMADevices.