power: bq24257: Add over voltage protection setting support

A new optional device property called "ti,ovp-voltage" is introduced to
allow configuring the input over voltage protection setting.

This commit also adds the basic sysfs support for custom properties
which is being used to allow userspace to read the current ovp-voltage
setting.

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>