brcmfmac: Create p2p0 netdev via module variable.

Add module variable with which a p2p0 netdev can be created. This
netdev can be used by wpa-supplicant to configure and set up the
p2p client/GO.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>