soc: arc: synopsys: move secure timer0 definition to DT

Use Devicetree to describe secure timer0 instead of hardcoding values in
<soc.h>.

DT files have been structured to match the following requirements: In
case of sectimer0 - it's should be only enabled for:

- emsdp_em7d_esp.dts
- em_starterkit_em7d.dts
- nsim_sem_mpu_stack_guard.dts
- nsim_sem.dts

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>