staging: omap-thermal: update documentation of omap-bandgap.h

This patch updates the existing data structures for omap bandgap,
inside omap-bandgap.h.

TODO: remove unused fields.
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>