srd: Add National LM75 protocol decoder.

This shall support the original National LM75 I2C-based temperature sensor,
as well as various compatible devices of other vendors, such as the
Fairchild FM75, Dallas DS75, and so on.

Currently only 9bit resolution is supported, and not all commands are
decoded, yet. The register contents and temperatures are decoded
correctly though, and can be output in both Celsius and Kelvin.