aspeed/smc: improve segment register support

The HW does not enforce all the rules in the specs and allows a few
"curious" setups like zero size segments and overlaps. So change the
model to be in sync but keep the warnings which are always interesting
for debug.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 1480434248-27138-13-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>