power: bq24257: Add input DPM voltage threshold setting support

A new optional device property called "ti,in-dpm-voltage" is introduced
to allow configuring the input voltage threshold for the devices'
dynamic power path management (DPM) feature. In short, it can be used to
prevent the input voltage from dropping below a certain value as current
is drawn to charge the battery or supply the system.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>