thermal: msm_tsens: add calibration for Tsens

Add calibration for Tsens temperature calculation;
Rewrite the conversion functions between temperature degree and ADC code;
Set TSENS_CONFIG bits (bits 29:28 of TSENS_CNTL) to '01'
because this setting is found to be optimal.

Change-Id: Ie40abb1f11c43b60b9a5a58eb84d360b190a2d02
Signed-off-by: Yan He <yanhe@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>