staging: brcm80211: brcmfmac: add debugfs, display wi-fi statistics

Add debugfs to display wi-fi profile/statistics.
Initially some profile parameters such as dtim_period,
beacon_int will be seen at
/sys/kernel/debug/ieee80211/phy#/netdev:eth#.
This will provide users with current connection status.

Signed-off-by: Nohee Ko <noheek@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>