hw/arm/virt: Fix non-secure flash mode

hw/arm/virt: Fix non-secure flash mode

Using the whole 128 MiB flash in non-secure mode is not working because
virt_flash_fdt() expects the same address for secure_sysmem and sysmem.
This is not correctly handled by caller because it forwards NULL for
secure_sysmem in non-secure flash mode.

Fixed by using sysmem when secure_sysmem is NULL.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Message-id: 20190712075002.14326-1-david.engraf@sysgo.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>