media: i2c: adv7180: Add support for V4L2_CID_LINK_FREQ

For CSI2 receivers that need to know the link frequency,
add it as a control to the driver.
Interlaced modes are 216Mbp/s or 108MHz, whilst going through
the I2P to deinterlace gives 432Mb/s or 216MHz.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>