dma: Add SoF and EoF debugging to ipu_idmac.c, minor cleanup

Add Start-of-Frame and End-of-Frame debugging to ipu_idmac.c, in the
future it might also be needed for the actual video processing in
mx3-camera, at which point, the ISRs will have to be transferred to
mx3_camera.c, for which ipu_irq_map() and ipu_irq_unmap() functions will
have to be exported.

Also simplify a couple of pointer-dereferences.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>