cfg80211: support ieee80211-freq-limit DT property

This patch adds a helper for reading that new property and applying
limitations of supported channels specified this way.
It is used with devices that normally support a wide wireless band but
in a given config are limited to some part of it (usually due to board
design). For example a dual-band chipset may be able to support one band
only because of used antennas.
It's also common that tri-band routers have separated radios for lower
and higher part of 5 GHz band and it may be impossible to say which is
which without a DT info.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
[add new function to documentation, fix link]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>