power: supply: max17042_battery: Add support for the TTE_NOW prop

The max170{42,47,50,55} family of fuel gauges all provide time-to-empty
estimation. As such, let's export this as a property.

Signed-off-by: Geordan Neukum <gneukum1@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>