Use mutex to arbitrate I2C buses.
Use mutex to arbitrate I2C buses.

We now have a task contantly polling temperature sensors, so we need to
use mutex to arbitrate I2C buses.

Signed-off-by: Vic Yang <victoryang@chromium.org>

BUG=chrome-os-partner:7491
TEST=temps, i2cscan, and i2cread all work fine.

Change-Id: I1360afb22d98b47da3da0820c95df45c15056f82

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