Added low level modem drivers.

BUG=chromium:272493
TEST=run_all_tests.py

Change-Id: I5a33efd09403846215409de94e1f402ce1c80fbf
Reviewed-on: https://chromium-review.googlesource.com/182851
Reviewed-by: Byron Kubert <byronk@chromium.org>
Tested-by: Byron Kubert <byronk@chromium.org>
Commit-Queue: Byron Kubert <byronk@chromium.org>
4 files changed