adp5061: New driver for ADP5061 I2C battery charger

adp5061: New driver for ADP5061 I2C battery charger

This patch adds basic support for Analog Devices I2C programmable linear
battery charger.

With this driver, some parameters can be read and configured such as:
* trickle charge current level (PRECHARGE_CURRENT)
* trickle charge voltage threshold (VOLTAGE_MIN)
* weak charge threshold (VOLTAGE_AVG)
* constant current (CONSTANT_CHARGE_CURRENT)
* constant charge voltage limit (CONSTANT_CHARGE_VOLTAGE_MAX)
* battery full (CAPACITY_LEVEL)
* input current limit (INPUT_CURRENT_LIMIT)
* charger status (STATUS)
* battery status (CAPACITY_LEVEL)
* termination current (CHARGE_TERM_CURRENT)

Datasheet:
http://www.analog.com/media/en/technical-documentation/data-sheets/ADP5061.pdf

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>