volteer: Use the EC version of battery.c

Instead of using a copy, bring in the file from the EC tree. Also add
the baseboard charger.c file so that it builds.

BUG=b:175434113
TEST=make BOARD=volteer -j30
build zephyr for volteer

Change-Id: I52b006c4b73c399eecc5571b7f864b30d8bf9924
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/zephyr-chrome/+/2613607
Commit-Queue: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Keith Short <keithshort@chromium.org>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
2 files changed