[media] smiapp-pll: Create a structure for OP and VT limits

OP and VT limits have identical fields, create a shared structure for
both.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>