MFC r318005:

[iwm] Add basic powermanagement support via ifconfig wlan0 powersave.

* The DEVICE_POWER_FLAGS_CAM_MSK flag was removed in the upstream iwlwifi
in Linux commit ceef91c89480dd18bb3ac51e91280a233d0ca41f.

* Add sc_ps_disabled flag to struct iwm_softc, which corresponds to
mvm->ps_disabled in struct iwl_mvm in Linux iwlwifi.

* Adds a hw.iwm.power_scheme tunable which corresponds to the power_scheme
module parameter in Linux iwlwifi. Set this to 1 for completely
disabling power management, 2 (default) for balanced powermanagement,
and 3 for lowerpower mode (which does dtim period skipping).

* Imports the constants.h file from iwlwifi as if_iwm_constants.h.

* This doesn't allow changing the powermanagement setting while connected,
also one can only choose between enabled and disabled powersaving with
ifconfig (so switching between balanced and low-power mode requires
rebooting to change the tunable).

* After any changes to powermanagement (i.e. "ifconfig wlan0 powersave" to
enable powermanagement, or "ifconfig wlan0 -powersave" for disabling
powermanagement), one has to disconnect and reconnect to the accespoint
for the change to take effect.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import OpenBSD's iwm WiFi driver for Intel 3160/7260/7265.

There are still several bugs, but I've been using it for a while now.
Thanks to all the testers and to Adrian for his help with this
driver.

This driver isn't connected to the build yet, but it will be soon.

There's no MFC planned because the driver isn't very stable yet.

Reviewed by: adrian
Obtained from: https://github.com/rpaulo/iwm
Tested by: adrian, gjb, dumbbell (others that I forgot).
Relnotes: yes