power: supply: twl4030-charger: add writable INPUT_CURRENT_LIMIT property

Currently, the twl4030 charger defines its own max_current by directly
creating sysfs nodes. It should use the input_current_limit property
which is e.g. used by the bq24257 driver.

This patch adds the input_current_property with the same semantics as
the max_current property. The code to manage the max_current property
is removed by a separate patch.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>