arm64: Fix enabling CPU features

Previously when enabling CPU feature we assumed the no check function
means the feature was unconditionally enabled. When adding support to
disable features on boot this check was incorrectly partially left in
place. As all current features have a check function this meant all
features were disabled.

Fix this by restoring the previous behaviour while also allowing the
user to disable the feature.

Reviewed by: emaste
Fixes: 4bc68fa98f68 ("arm64: Support managing features from loader")
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D52579