dmaengine: dw: update MAINTAINERS file

This is a follow up to the previously done changes in the layout of the driver
files. We now have an additional file include/linux/dma/dw.h which is missed in
the MAINTAINERS data base.

Fixes: 3d588f83 (dmaengine: dw: split dma-dw.h to platform and private parts)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>