brcmfmac: expose firmware config files through modinfo

Apart from a firmware binary the chip needs a config file used by the
FW. Add the config files to modinfo so that they can be read by
userspace.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201124120018.31358-1-matthias.bgg@kernel.org