staging: ti-soc-thermal:Introduce ti_bandgap_get_trend function for OMAP5
The patch adds ti_bandgap_get_trend function. This is specific
to OMAP5 for now it computes the trend from the temp values stored
in the hardware history buffer.
Formula: (T1 - T2) / P.
Where:
T1: Last read valid temperature.
T2: Last but one read valid temperature.
P: Update Interval.