riscv: dts: drop microchip from dts filenames

The original names picked for the DT doesn't match Linux's naming scheme
and it was renamed there a while ago. Rename it in U-Boot to allow
easily syncing dts between the two projects.

Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Padmarao Begari <padmarao.begari@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>