aspeed: add support for the SMC segment registers

The SMC controller on the Aspeed SoC has a set of registers to
configure the mapping of each flash module in the SoC address
space. Writing to these registers triggers a remap of the memory
region and the spec requires a certain number of checks before doing
so.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Message-id: 1474977462-28032-7-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
