power: bq28400: Add module parameter to fake battery SOC

The module parameter allows faking the SOC to the
specified value and prevents reporting the true SOC.

Example to fake the SOC to 75%
echo 75 > /sys/module/bq28400_battery/parameters/fake_battery

Change-Id: I5963a407b4ae847a5accfeb1dd8c478817ff0a57
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>