#16 CMSIS-SVD cluster level support

[Jonas Meyer]

CMSIS SVD supports grouping registers into clusters. This is not supported by embsysregview.
https://www.keil.com/pack/doc/CMSIS/SVD/html/group__cluster_level__gr.html

I'm currently modifying a SVD file and would very much like to use this grouping mechanism. If this grouping would be represented in the gui would be extra nice.

Currently it seems all registers are treated as arrays of registers. So if there is only a single element in the array of registers the same "folder" has to be opened twice. Instead the folder depth should depend on how deep the nesting actually is, since the latest SVD version allows nesting clusters arbitrarily deep.