media: imx208: Add imx208 camera sensor driver

Add a V4L2 sub-device driver for the Sony IMX208 image sensor.
This is a camera sensor using the I2C bus for control and the
CSI-2 bus for data.

[Sakari Ailus: Rename sensor async register function to make it compile,
	       use exposure_max and wrap a few long lines.]

Signed-off-by: Ping-Chung Chen <ping-chung.chen@intel.com>
Signed-off-by: Yeh, Andy <andy.yeh@intel.com>
Signed-off-by: Shawn Tu <shawnx.tu@intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>