riscv: dts: microchip: remove soc vendor from filenames

Having the SoC vendor both as the directory and in the filename adds
little. Remove microchip from the filenames so that the files will
resemble the other directories in riscv (and arm64). The new names
follow a soc-board.dts & soc{,-fabric}.dtsi pattern.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220509142610.128590-4-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>