[media] smiapp: Allow using external clock from the clock framework

Instead of providing a function in platform data, allow also providing the
name of the external clock and use it through the clock framework.

Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>