power: supply: qti_battery_charger: add support to set fake SOC for battery

Setting a fake battery SOC (state of charge) is useful to test
different mitigation levels. Add support for it by exposing
fake_soc under qcom-battery class which can be set only when
CONFIG_QTI_PMIC_GLINK_CLIENT_DEBUG is enabled.

To set fake_soc of 50%,

 echo 50 > /sys/class/qcom-battery/fake_soc

To clear fake_soc so that actual battery SOC can be displayed,
any value other than 0-100 can be set to fake_soc.

 echo -1 > /sys/class/qcom-battery/fake_soc

Change-Id: Ibff337fb4da9d4e9aadbe148f3eecca9e2d69604
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>