wl12xx: add auto-arp support

The auto-arp feature of wl12xx allows the firmware to automatically
response to arp requests asking for its ip.

in order to use it, we configure the arp response template and
enable the corresponding bit in wl1271_acx_arp_filter (along with
passing its ip)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>