Add a System View Description (SVD) file

[stnolting]

This PR is an attempt to provide a SVD file for the NEORV32. The idea comes from the project's gitter chat.
The file will be sw/svd/neorv32.svd.

The file format is compatible to ARM's CMSIS-SVD format (Apache-2.0 License ).

• ARM/Keil documentation: https://www.keil.com/pack/doc/CMSIS/SVD/html/index.html
• CMSIS 4.4 example: https://github.com/ARM-software/CMSIS/blob/master/CMSIS/SVD/ARM_Example.svd
• RISC-V (SiFive) SVD example: https://github.com/riscv-rust/e310x/blob/master/e310x.svd
• Syntax verified using SVDConv utility
• Verified C-header also generate by SVDConv utility

The NEORV32 SVD file is (located in) sw/svd/neorv32.svd. Right now, it just contains a minimal CPU description and the registers of the PWM and CFS peripherals. I think it makes sense to add all peripherals to that file even though certain modules might not be implemented in a specific setup.

This PR is open for discussion. Help is highly appreciated!

[stnolting]

✔️ Thanks @emb4fun and gitter user Irvise for the feedback.