MAINTAINERS: ioat: remove dmaengine susbstem files

[1] mentions the IOAT entry contains dmaengine subsystem file. So update
the entry and remove the dmaengine files

1: https://lwn.net/Articles/842415/

Link: https://lore.kernel.org/r/20210117070229.2537866-1-vkoul@kernel.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>