media: staging/imx: refactor imx media device probe

Refactor and move media device initialization code to a new common
module, so it can be used by other devices, this will allow for example
a near to introduce imx7 CSI driver, to use this media device.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>