FROMLIST: psi: rename psi fields in preparation for psi trigger addition

Renaming psi_group structure member fields used for calculating psi totals
and averages for clear distinction between them and trigger-related fields
that will be added next.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>

(not upstream yet, latest version published at: https://lore.kernel.org/patchwork/patch/1052416/

)

Bug: 127712811
Bug: 129157727
Test: lmkd in PSI mode
Change-Id: I7aaadfc558950b54b02a051d63e508e8fe233b49
Signed-off-by: Suren Baghdasaryan <surenb@google.com>