max17042_battery: Support regmap to access device's registers

This patch makes max17042 fuelguage driver uses regmap API to access its
device's registers.

Signed-off-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>