aspeed/smc: set the number of flash modules for the FMC controller

Add a new configuration field at the board level and propagate the
value using the "num-cs" property of the FMC controller model.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 1480434248-27138-14-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>