Add WC support for arm64 in the ENA driver

As the pmamp_change_attr() is public on arm64 since r351131, it can be
used on the arm64 to map memory range as with the write combined
attribute.

It requires the driver to use generic VM_MEMATTR_WRITE_COMBINING flag
instead of the x86 specific PAT_WRITE_COMBINING.

Differential Revision: https://reviews.freebsd.org/D21931
Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.