[media] V4L2: add a v4l2-clk helper macro to produce an I2C device ID

To obtain a clock reference consumers supply their device object to the
V4L2 clock framework. The latter then uses the consumer device name to
find a matching clock. For that to work V4L2 clock providers have to
provide the same device name, when registering clocks. This patch adds
a helper macro to generate a suitable device name for I2C devices.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>