power: bq24257: Add basic support for bq24250/bq24251

This patch adds basic support for bq24250 and bq24251 which are very
similar to the bq24257 the driver was originally written for. Basic
support means the ability to select a device through Kconfig, DT and
ACPI, an instance variable allowing to check which chip is active, and
the reporting back of the selected device through the model_name power
supply sysfs property.

This patch by itself is not sufficient to actually use those two added
devices in a real-world setting due to some feature differences which
are addressed by other patches in this series.

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