iwlwifi: mvm: add low-latency framework

For various traffic use cases, we want to be able to treat multi-
channel scenarios differently. Introduce a low-latency framework
that currently only has a debugfs file to enable low-latency mode,
but can later be extended.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>