hw/arm: Wire up BMC boot flash for npcm750-evb and quanta-gsj

hw/arm: Wire up BMC boot flash for npcm750-evb and quanta-gsj

This allows these NPCM7xx-based boards to boot from a flash image, e.g.
one built with OpenBMC. For example like this:

IMAGE=${OPENBMC}/build/tmp/deploy/images/gsj/image-bmc
qemu-system-arm -machine quanta-gsj -nographic \
	-drive file=${IMAGE},if=mtd,bus=0,unit=0,format=raw,snapshot=on

Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200911052101.2602693-12-hskinnemoen@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>