aspeed: add support for the romulus-bmc board

The Romulus machine is an OpenPOWER system with an AST2500 SoC for
the BMC and a POWER9 chip for the host. It does not make much
difference for qemu a part from the fact that the FMC controller has
two SPI flash module.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 1480434248-27138-8-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>