[service] Assign realtime budget for DSME service

DSME need RT budget to use SCHED_FIFO

By default systemd places all system services into their own control groups in the "cpu" hierarchy. If a cgroup is created it needs an explicit, absolute RT time budget assigned, or otherwise RT is not available to any process in the group, and an attempt to acquire it will fail with EPERM. systemd will not assign any RT time budgets to the "cpu" cgroups it creates, simply because there is no feasible way to do that, since the budget needs to be specified in absolute time units and comes from a fixed pool.