Add generator for CMSIS SVD

[raffi-g]

This PR adds a generator for CMSIS System View Description files (see here.

As described in #12, in a strictly correct SVD file, the registers are separated into peripherals. However, since this information is not provided in the current register definition, I've simply put them all into one peripheral for now (the name of which is defined as peripheral_name in the config file). Additionally the part_version and a description also have to be given in the config file.

I've checked the output using CMSIS's SVDConv tool.