Platform/Socionext/DeveloperBoxMm: increase FV size

Due to recent changes in core EDK2, the DeveloperBoxMM standalone MM
build no longer fits when built in DEBUG or NOOPT mode. So increase
the FD size a little bit - we still have some spare room available in
the FIP image.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>