iwlwifi: mvm: add bcast_filtering debugfs entries

Allow reading and setting bcast filtering configuration
through debugfs.

By default, mvm->bcast_filters is used for setting
the bcast filtering configuration (these filters
will be configured for each associated station).

For testing purposes, allow overriding this configuration,
and setting the bcast filtering configuration manually.
The following debugfs keys can be used:
* bcast_filtering/override - use debugfs values instead
	of default configuration
* bcast_filtering/filters - set filters (+ attributes)
* bcast_filtering/macs - per-mac bcast filtering
	configuration (policy + attached filters)

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>