power: bq24257: Allow input current limit sysfs access

This patch allows reading and writing of the input current limit through
the power supply's input_current_limit sysfs property. This allows
userspace to see what charger was detected (if the D+/D- USB signal-
based charger type detection is enabled) and to re-configure the maximum
current drawn from the external supply at runtime based on system-level
knowledge or user input. Note that upon charger disconnection and
re-connection the limit configured through firmware becomes active again
(or the D+/D- USB signal-based charger detection will be run again).

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