aspeed/smc: Do not map disabled segment on the AST2600

aspeed/smc: Do not map disabled segment on the AST2600

The segments can be disabled on the AST2600 (zero register value).
CS0 is open by default but not the other CS. This is closing the
access to the flash device in user mode and forbids scanning.

In the model, check the segment size and disable the associated region
when the value is zero.

Fixes: bcaa8ddd081c ("aspeed/smc: Add AST2600 support")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20191119141211.25716-12-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>