oak: Fix PSU callback

This patch does two things:
1. Gets rid of Oak's boilerplate psu.c file and uses the generic callback.
2. Corrects the output. Oak is a Chromebook so it should be "battery".

TEST="mosys psu type" on oak returns "battery" now

Signed-off by: David Hendricks <dhendrix@chromium.org>
Change-Id: I064383f7d1a5aef13537ed83aa02416fdddd6a4d
Reviewed-on: https://chromium-review.googlesource.com/315746
Commit-Ready: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Shawn N <shawnn@chromium.org>
3 files changed