[media] vivid: Add an option to configure the maximum number of devices

The vivid driver currently has a hard-coded limit of 64 devices,
however there's nothing that prevents the creation of even more devices.
This commit adds a new driver option (which defaults to 64) to
allow this maximum number to be configurable.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>