hwmon: Add driver for ADT7411 voltage and temperature sensor

Add basic support for the ADT7411. Reads out all conversion results (via I2C,
SPI yet missing) and allows some on-the-fly configuration. Tested with a
custom board.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Jean Delvare <khali@linux-fr.org>