dt-bindings: document common IEEE 802.11 frequency limit property

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>;

	};

};