gta01_battery: generalise the driver, add capacity estimation

Check if machine-specific hooks exist before calling, this allows to use
this driver with gta02 or any other device that is able to provide battery
voltage readings.

Report an estimate of capacity percentage left, based on discharge curves.
Strictly speaking, this is a violation of power_supply class specs but
since this is the only driver for dumb batteries and userspace lacks any
library to do the estimation itself, we have to stuff it here.

Please keep in mind that this estimation is by no means accurate, usually
cell phones have only 5 bars to represent charge status.

Credit goes to Joerg Reisenweber and SpeedEvil for helping with gathering
and analysing the data.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

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