brcmfmac: add "reset" debugfs entry for testing reset

This is a trivial debugfs entry for triggering reset just like in case
of firmware crash. It works by writing 1 to it:
echo 1 > reset

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>