iwlwifi: mvm: Add support for Low Power RX

To improve power consumption in idle associated mode FW may lower
RX power. This low linearity mode is acceptable for listening low rate
RX such as beacons and groupcast. The driver enables LPRX only if PM
is enabled and associated AP's beacon TX rate is 1Mbps or 6Mbps.
LPRX RSSI threshold is used to limit a range where LPRX is applied.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>