charger: add sy21612 buck-boost converter driver

SY21612 is buck-boost converter with selectable source/sink mode.

BRANCH=none
BUG=none
TEST=none

Signed-off-by: Rong Chang <rongchang@chromium.org>
Change-Id: I71248eedd9be775790d71010f69dfae41cd64a27
Reviewed-on: https://chromium-review.googlesource.com/673964
Reviewed-by: Benson Leung <bleung@chromium.org>
4 files changed