arm64: Add a DBM errata workaround
Add a workaround for Cortex-A55 erratum 1024718 and Cortex-A510 erratum
- Both errata are related to the hardware handling of the dirty
field in page tables and can be worked around by disabling this feature.
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47809