drm: atmel-hlcdc: Add dependency on ARM

The atmel-hlcdc driver selects DRM_GEM_CMA_HELPER which makes use of
symbols only available when HAVE_DMA_ATTRS is selected.
Add a dependency on the ARM architecture which select this option.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>