hw/arm: Add npcm7xx emc model

hw/arm: Add npcm7xx emc model

This is a 10/100 ethernet device that has several features.
Only the ones needed by the Linux driver have been implemented.
See npcm7xx_emc.c for a list of unimplemented features.

Reviewed-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Avi Fishman <avi.fishman@nuvoton.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Doug Evans <dje@google.com>
Message-id: 20210218212453.831406-3-dje@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>