TPS65911: Comparator: Add comparator driver

This driver adds functionality to the tps65911 chip driver.

Two of the comparators are configurable by software and measures
VCCS voltage to detect high or low voltage scenarios.

Signed-off-by: Jorge Eduardo Candelaria <jedu@slimlogic.co.uk>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>