power: bq24257: Add various device-specific sysfs properties

This patch adds support for enabling/disabling optional device specific
features through sysfs properties at runtime.

* High-impedance mode enable/disable
* Sysoff enable/disable

Refer to the respective device datasheets for more information:

http://www.ti.com/product/bq24250
http://www.ti.com/product/bq24251
http://www.ti.com/product/bq24257



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