hw/arm/aspeed: Add machine properties to define the flash models

hw/arm/aspeed: Add machine properties to define the flash models

Some machines don't have much differences a part from the flash model
being used. Introduce new machine properties to change them from the
command line.

For instance, to start the ast2500-evb machine with a different FMC
chip and a 64M SPI chip, use :

  -M ast2500-evb,fmc-model=mx25l25635e,spi-model=mx66u51235f

Cc: 郁雷 <yulei.sh@bytedance.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Tested-by: Lei YU <yulei.sh@bytedance.com>
Message-Id: <20200915054859.2338477-1-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>