Switch the arm64 VM_MEMATTR_DEVICE to nGnRE

Move device memory to a weaker type. The new device memory type allows
the system to acknowledge a write to a device before the write has
completed. This is inline with VM_MEMATTR_DEVICE on armv6/armv7.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D38945