iwlwifi: mvm: add LQM vendor command and notification

LQM stands for Link Quality Measurement. The firmware
will collect a defined set of statitics (see the
notification for details) that allow to know how busy
the medium is. The driver issues a request to the firmware
that includes the duration of the measurement (the firmware
needs to be on channel for that amount of time) and the
timeout (in case the firmware has a lot of offchannel
activities). If the timeout elapses, the firmware will
send partial results which are still valuable.
In case of disassociation / channel switch and alike, the
driver is in charge of stopping the measurements and the
firmware will reply with partial results.

The user space API for now is debugfs only and will be
implmemented in an upcoming patch.

Signed-off-by: Aviya Erenfeld <aviya.erenfeld@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>