ANDROID: GKI: dma-mapping.h: add Android ABI padding to a structure

Try to mitigate potential future driver core api changes by adding a
padding to struct dma_map_ops.

Based on a change made to the RHEL/CENTOS 8 kernel.

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Icbec584dee3716f341a7831a4810025f690eab4a