target/arm: Register second AddressSpace for secure v8M CPUs

If a v8M CPU supports the security extension then we need to
give it two AddressSpaces, the same way we do already for
an A profile core with EL3.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1503414539-28762-5-git-send-email-peter.maydell@linaro.org
