dt: power: bq24257-charger: Cover additional devices

Extend the bq24257 charger's device tree documentation to cover the
bq24250 and bq24251 devices as well feature additions.

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