[][openwrt][mt7988][tops][add cpu utilization fetching]

[Description]
Add cpu utilization fetching and refactor debugfs of trm and tops

Via reading debugfs file(/sys/kernel/debug/tops/trm/cpu-utilization)
to fetch each core's cpu utilization

[Release-log]
N/A

Change-Id: Iff4c965294c3054a54ba92263191856cd5c722c7
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7907085
11 files changed