soc: riscv: miv/mpfs: reorganize SoC folder

Merge both series into a new family: microchip_miv [1], moving them out
of riscv-privileged. Updated naming to stay closer to what vendor
announces on their website.

[1]: https://www.microchip.com/en-us/products/fpgas-and-plds/


     fpga-and-soc-design-tools/mi-v

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>