ipa: raspberrypi: Allow long exposure modes for imx477.

Update the imx477 CamHelper to use long exposure modes if needed.
This is done by overloading the CamHelper::GetVBlanking function to return a
frame length (and vblank value) computed using a scaling factor when the value
would be larger than what the sensor register could otherwise hold.

CamHelperImx477::Prepare is also overloaded to ensure that the "device.status"
metadata returns the right value if the long exposure scaling factor is used.
The scaling factor is unfortunately not returned back in metadata.

With the current imx477 driver, we can achieve a maximum exposure time of approx
127 seconds since the HBLANK control is read-only.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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