Extend the AD4000 series device tree documentation to also describe PulSAR devices. The single-channel series of PulSAR devices is similar to the AD4000 series except PulSAR devices sample at slower rates and don't have a configuration register. Because PulSAR devices don't have a configuration register, they don't support all features of AD4000 devices and thus fewer interfaces are provided to user space. Also, while AD4000 may have their SDI pin connected to SPI host MOSI line, PulSAR SDI pin is never connected to MOSI. Some devices within the PulSAR series are just faster versions of others.
