Add an IDC only arm64 icache sync function

When the IDC flag is set in the cache type register we don't need to
clean the data cache to the point of unification. Previously we
supported this flag being set only when the DIC flags was also set.
Add a new handler for when this is not the case.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation, Ampere (hardware)
Differential Revision: https://reviews.freebsd.org/D36296