[media] V4L: remove clock name from v4l2_clk API

All uses of the v4l2_clk API so far only register one clock with a fixed
name. This allows us to get rid of it, which also will make CCF and DT
integration easier.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>