Support more than 5 analog muxes

[ingobecker]

In the current design with a single shift register, a maximum of 5 analog multiplexers can be addressed(further limited by PCB design constraints to 4). To address more multiplexers it is required to add an additional shift register for every 5(or 4) additional analog multiplexers. Daisychaining additional shift registers allows to grow the number of supported analog muxes without the need for more microcontroller output pins.

• insert an additional shift register inbetween the microcontroller and the existing one
• add code to the Stm32Backend to address more than a single shift register