hdctools: servod: Add voltage, current, power readings of battery from EC.

BUG=chrome-os-partner:9353
TEST=manual
  sudo servod -c servo_link_overlay.xml
  dut-control lid_open dev_mode ppvar_vbat_mv ppvar_vbat_ma ppvar_vbat_mw

Change-Id: I9d5b2a0b4f1ef1c7689db7fd3b145374f7a3ef75
Reviewed-on: https://gerrit.chromium.org/gerrit/22155
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Tested-by: Todd Broch <tbroch@chromium.org>
Commit-Ready: Todd Broch <tbroch@chromium.org>
2 files changed