brcmfmac: introduce checkdied debugfs functionality

The checkdied functionality provides useful information for analyzing
firmware crashes. By exposing this information to a debugfs file users
can easily provide its content in bug reports. The functionality is
available only when CONFIG_BRCMDBG is selected.
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>