brcmfmac: introduce scheduled scan support

This change add support for NL80211 scheduled scan. This may be used to
offload scanning to the device, which may give the host opportunity to
sleep. The newer versions of wpa_supplicant have support for this
functionality.

Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>