target-arm: Add QOM property for Secure memory region

target-arm: Add QOM property for Secure memory region

Add QOM property to the ARM CPU which boards can use to tell us what
memory region to use for secure accesses. Nonsecure accesses
go via the memory region specified with the base CPU class 'memory'
property.

By default, if no secure region is specified it is the same as the
nonsecure region, and if no nonsecure region is specified we will use
address_space_memory.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>