hw/mem: Stubbed out NPCM7xx Memory Controller model

This just implements the bare minimum to cause the boot block to skip
memory initialization.

Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Message-id: 20200911052101.2602693-10-hskinnemoen@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>