nissa: Add a board check for external power present
nissa: Add a board check for external power present

Add a check for the presence of external power.
Each charger is checked to see if a charger is connected.

BUG=b:214478293
TEST=zmake configure -b nivviks; flash and run, check for ac=1
BRANCH=none

Signed-off-by: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>
Change-Id: I7da723b693f55e8a5d3590ac88ce10f4e4f732f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3403832
Reviewed-by: Peter Marheine <ef0b2c492edde94778d7465bfbaeed52c5be6858@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec