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>
diff --git a/net/wireless/Makefile b/net/wireless/Makefile
index 4c9e39f..95b4c09 100644
--- a/net/wireless/Makefile
+++ b/net/wireless/Makefile
@@ -11,6 +11,7 @@
 
 cfg80211-y += core.o sysfs.o radiotap.o util.o reg.o scan.o nl80211.o
 cfg80211-y += mlme.o ibss.o sme.o chan.o ethtool.o mesh.o ap.o trace.o ocb.o
+cfg80211-$(CONFIG_OF) += of.o
 cfg80211-$(CONFIG_CFG80211_DEBUGFS) += debugfs.o
 cfg80211-$(CONFIG_CFG80211_WEXT) += wext-compat.o wext-sme.o
 cfg80211-$(CONFIG_CFG80211_INTERNAL_REGDB) += regdb.o