staging: ti-soc-thermal: Add get_trend support

Patch adds get_trend functionality for OMAP Bandgap thermal devices.

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>