hw/ssi: Make flash size a property in NPCM7XX FIU

hw/ssi: Make flash size a property in NPCM7XX FIU

This allows different FIUs to have different flash sizes, useful
in NPCM8XX which has multiple different sized FIU modules.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@linaro.org>
Message-id: 20250219184609.1839281-4-wuhaotsh@google.com
[PMM: flash_size must be a uint64_t to build on 32-bit hosts]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>