Documentation: power: bq24257: Document exported sysfs entries

Document the settings exported by bq24257 charger driver through sysfs
entries:
- ovp_voltage
- in_dpm_voltage
- high_impedance_enable
- sysoff_enable

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