scmi_clock: Update comments about source of event

Created by: BY1994

Hi, I want to change the comments in scmi_clock slightly. In current status, event source is missing for the first comment. When I looked module/scmi_perf/src/mod_scmi_perf.c file (2304 - 2310 lines), the comments helped me a lot to understand the codes. So I want to suggest changing the comments as the same format. Any feedback is welcome including suggestion of commit message.