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.

Signed-off-by: J Keerthy <j-keerthy@ti.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>