Add the lm75 i2c digital temperature sensor driver.

This driver supports the low and high precision models (9 and 11 bits) and
it will auto-detect the both variants.

The driver expose the temperature registers (actual temperature, shutdown
and hysteresys temperature) and also the configuration register.

It was tested on FDT systems: RPi, BBB and on non-FDT systems: AR71xx, with
both, hardware i2c controllers (when available) and gpioiic(4).

This provides a simple and cheap way for verifying the i2c bus on embedded
systems.