Common IEEE 802.11 properties

This provides documentation of common properties that are valid for all wireless
devices.

Optional properties:
 - ieee80211-freq-limit : list of supported frequency ranges in KHz. This can be
	used for devices that in a given config support less channels than
	normally. It may happen chipset supports a wide wireless band but it is
	limited to some part of it due to used antennas or power amplifier.
	An example case for this can be tri-band wireless router with two
	identical chipsets used for two different 5 GHz subbands. Using them
	incorrectly could not work or decrease performance noticeably.

Example:

pcie@0,0 {
	reg = <0x0000 0 0 0 0>;
	wifi@0,0 {
		reg = <0x0000 0 0 0 0>;
		ieee80211-freq-limit = <2402000 2482000>,
				       <5170000 5250000>;
	};
};