hw/arm/virt: Make first flash device Secure-only if booting secure

hw/arm/virt: Make first flash device Secure-only if booting secure

If the virt board is started with the 'secure' property set to
request a Secure setup, then make the first flash device be
visible only to the Secure world.

This is a breaking change, but I don't expect it to be noticed
by anybody, because running TZ-aware guests isn't common and
those guests are generally going to be booting from the flash
and implicitly expecting their Non-secure guests to not touch it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1455288361-30117-5-git-send-email-peter.maydell@linaro.org