brcmfmac: trigger memory dump upon firmware halt signal

PCIe dongle firmware signals a halt/trap through mailbox interrupt.
Trigger a memory dump upon receiving such signal could help to provide
useful information for issue debug.

Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>