[media] v4l2-clk: add new macro for v4l2_clk_name_of()

This macro is used to generate an OF string for a v4l2 clock.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>