cnss2: Add support to download regdb file to firmware

Download regdb file as another type of board data file to firmware
through QMI. Since the file is optional on certain platforms and also
in order to keep backward compatibility, continue to download normal
board data file if regdb file is not present on the file system.

Change-Id: I14456724e4f65d9d04f134e103b4fc3d2301ccbe
Signed-off-by: Yue Ma <yuem@codeaurora.org>