[media] V4L: Add driver for s5k6a3 image sensor

This patch adds subdev driver for Samsung S5K6A3 raw image sensor.
As it is intended at the moment to be used only with the Exynos
FIMC-IS (camera ISP) subsystem it is pretty minimal subdev driver.
It doesn't do any I2C communication since the sensor is controlled
by the ISP and its own firmware.
This driver, if needed, can be updated in future into a regular
subdev driver where the main CPU communicates with the sensor
directly.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>