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.
Backports commit 9e273ef2174d7cd5b14a16d8638812541d3eb6bb from qemu