aspeed: use first FMC flash as a boot ROM

Create a ROM region, using the default size of the mapping window for
the CE0 FMC flash module, and fill it with the flash content.

This is a little hacky but until we can boot from a MMIO region, it
seems difficult to do anything else.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Message-id: 1483979087-32663-11-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
