dt-bindings: fpga: fpga-region: Convert to sugar syntax

Using overlay sugar syntax makes the DTS files easier to read (and
write).

While at it, fix two build issues:
  - "/dts-v1/" and "/plugin/" must be separate statements.
  - Add a missing closing curly brace.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Moritz Fischer <mdf@kernel.org>