arm64: bus: Add 36-bit address mask for use in bus space allocations

Obtained from: Juniper Networks, Inc.
MFC after: 1 week
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44764

(cherry picked from commit 5484c3d6eb31a78f07ed5e66862a1e7d3b5225b6)