Use pmap_preboot_map_attr() directly in arm_devmap_bootstrap()
instead of hiding behind pmap_map_chunk(). It's not longer needed
after old pmap-v6 code was removed.

For compatibility with ARM_ARCH < 6, define PTE_DEVICE in devmap.c
file. Certainly, it would be nice if VM_MEMATTR_DEVICE could be used
even for ARM_ARCH < 6.