MAINTAINERS: Step down from a co-maintaner of DW DMAC driver

As discussed at ELCE 2017 there is little to anticipate from me in the
future with regard to the driver, and since I have many things to keep
an eye on, I would like to step down to simple designated reviewer.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>