[media] smiapp: Get clock rate if it's not available through DT

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: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>