[media] V4L: mx2_camera: remove unsupported i.MX27 DMA mode, make EMMA mandatory

The i.MX27 DMA support was introduced with the initial commit of this
driver and originally created by me. However, I never got this stable
due to the racy DMA engine and used the EMMA engine instead. As the DMA
support is most probably unused and broken in its current state, remove
it. EMMA becomes the only supported mode on i.MX27.

This also helps us get rid of another user of the legacy i.MX DMA
support and remove the dependency on ARCH_MX* macros as these are
scheduled for removal.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
[g.liakhovetski@gmx.de: remove unused goto]
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>