power: supply: bq27xxx: Support CHARGE_NOW for bq27z561/bq28z610/bq34z100

Currently REG_NAC (nominal available capacity) is mapped to
power-supply's CHARGE_NOW property. Some chips do not have
REG_NAC and do not expose CHARGE_NOW at the moment. Some
bq27xxx chips also have another register REG_RM (remaining
capacity). The difference between REG_NAC and REG_RM is load
compensation.

This patch adds register information for REG_RM for all
supported fuel gauges. On systems having REG_NAC it is
ignored, so behaviour does not change. On systems without
REG_NAC, REG_RM will be used to provide CHARGE_NOW
functionality.

As a result there are three more chips exposing CHARGE_NOW:
bq27z561, bq28z610 and bq34z100

Signed-off-by: Hermes Zhang <chenhuiz@axis.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>