cam-service: Enable pthread creation on Zephyr

1. Define a top level array of thread stack memory regions on Zephyr.
   The count of threads is CAM_SERVICE_MAX_CONN + 1 which means one
   dispatcher thread and CAM_SERVICE_MAX_CONN stream connection
   process threads.
2. Define a non-null pthread attribute while creating new thread on
   Zephyr.

Signed-off-by: Hongda Deng <hongda.deng@arm.com>