kernel: system_work_q: make `k_work_queue_config cfg` as `static const`

Make `k_work_queue_config cfg` as `static const` to enable
compile-time instantiation instead of runtime allocation.
This modification saves substantial flash memory but has system-wide
effects that should be considered.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>