Obtain the clock rate from the clock framework if it's not available
through DT. The assumption is that the parent device (camera module)
defines the rate as clock control is a part of the power on and power off
sequences --- which are camera module specific.

Also use the clock rate from DT if no clock is provided.
Signed-off-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>