V4L2 driver updates (#1393)

* BCM2835-V4L2: Correct ISO control and add V4L2_CID_ISO_SENSITIVITY_AUTO

https://github.com/raspberrypi/linux/issues/1251



V4L2_CID_ISO_SENSITIVITY was not advertising ISO*1000 as it should.
V4L2_CID_ISO_SENSITIVITY_AUTO was not implemented, so was taking
V4L2_CID_ISO_SENSITIVITY as 0 for auto mode.
Still accepts 0 for auto, but also abides by the new parameter.

Signed-off-by: Dave Stevenson <6by9@users.noreply.github.com>

* BCM2835-V4L2: Add a video_nr parameter.

Adds a kernel parameter "video_nr" to specify the preferred
/dev/videoX device node.
https://www.raspberrypi.org/forums/viewtopic.php?f=38&t=136120&p=905545



Signed-off-by: Dave Stevenson <6by9@users.noreply.github.com>

* BCM2835-V4L2: Add support for multiple cameras

Ask GPU on load how many cameras have been detected, and
enumerate that number of devices.
Only applicable on the Compute Module as no other device
exposes multiple CSI2 interfaces.

Signed-off-by: Dave Stevenson <6by9@users.noreply.github.com>

* BCM2835-V4L2: Add control of the overlay location and alpha.

Actually do something useful in vidioc_s_fmt_vid_overlay and
vidioc_try_fmt_vid_overlay, rather than effectively having
read-only fields.

Signed-off-by: Dave Stevenson <6by9@users.noreply.github.com>

* BCM2835-V4L2: V4L2-Compliance failure fix

VIDIOC_TRY_FMT was failing due to bytesperline not
being set correctly by default.

Signed-off-by: Dave Stevenson <6by9@users.noreply.github.com>

* BCM2835-V4L2: Make all module parameters static

Clean up to correct variable scope

Signed-off-by: Dave Stevenson <6by9@users.noreply.github.com>