[media] mem2mem_testdev: set timestamp_type and add debug param

While testing v4l2-ctl I noticed that this m2m driver didn't set timestamp_type
and that it spammed the kernel log with debug messages. Set timestamp_type
correctly and add debug module option to enable debug messages.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>