driver-74hc595 - Embedded async 74hc595 driver

An embedded async driver for 74hc595 (compatible) shift registers.

For efficient data transfer, an SPI bus peripheral is used. The SPI bus should be configured in SPI Mode 0 (clock idle state low, data sampled on rising edge). Because the 74hc595 does not have a CS line, the SPI bus cannot be shared with other devices (at least not without additional circuitry).