Add support for arm64 nGnRE device memory

On arm64 we can select how strongly we order device memory. Currently
we use the strongest type of non-Gathering, non-Reordering, no Early
write acknowledgement. This is equivalent to VM_MEMATTR_SO in the 32-bit
arm code.

Create a new memory type to remove the no Early write acknowledgement
option to create a memory attribute that is equivalent to the arm
VM_MEMATTR_DEVICE.

Keep the the old nGnRnE memory as what we provide for VM_MEMATTR_DEVICE
until we can test nGnRE on more hardware. A method for dynamically
switching back may be needed as at least one vendor is known to have
broken nGnRE memory.

Sponsored by: Innovate UK