MLK-18450: mxc-jpeg: Fix JPEG decoder crash

Fixed decoder crasher by creating a structure to encompass the descriptors
and associated it to the jpeg device, one for each slot.
Removed mxc_jpeg_buf, as writing to its fields was causing
memory corruption sometimes (depending on how src_buf & dst_buf
got allocated, writing to one was overwriting the other).

For now, only slot 0 is used (no regression here, just a TODO).

Note that the decoded image looks ok only on QXP B0, not A0.

The decoder_test application will be patched in linux-test,
the old one should work too, but needed cleanup.

Signed-off-by: Mirela Rabulea <mirela.rabulea@nxp.com>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>