Add save/restore support to arcv2_timer_0/sys_clock

[zephyrbot]

Reported by Julien Delayen:

In order to restore the interrupts on the ARC core, the arcv2_timer_0
needs to support save and restore operations.

(Imported from Jira ZEP-1224)

[zephyrbot]

by Sharron LIU:

Verified passed via ZEP-1222.
No regression detected on arc v2 timer or system clock. Closing this.

[zephyrbot]

by Mark Linkmeyer:

Fixing incorrect priority