brcmfmac: add debugfs helper functions

This patch adds debugfs support to brcmfmac. It provide helper functions
to setup the debugfs folder structure for the driver, which has following
hierarchy:

<debugfs_mount>/brcmfmac/<dev_name>/

ie.: /sys/kernel/debug/brcmfmac/mmc0:0001:2/

The new source file provides functions to create and remove the two
folders and a function to retrieve the device-specific folder so files
can be created in it.

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>