V4L2 clock api

// V4L2 clocks 
// subdevices: 子设备  camera sensors,TV decoders and encoders
// need a clocksignal to be supplied by the system. Often this clock is supplied by the
// respective bridge device
// clock object 
// Common Clock Framework  并不适用所有架构 
v4l2_clk_register()
v4l2_clk_unregister()
v4l2_clk_get()
v4l2_clk_put()
v4l2_clk_enable()
v4l2_clk_disable()
v4l2_clk_get_rate()
v4l2_clk_set_rate()