nfp: change firmware directory layout

Move the current firmware files for Netronome Agilio CX SmartNICs into a
subdirectory and use symlinks in the main netronome directory to point
to this firmware. This is to prepare for future firmware additions for
the same cards that provide different functionality.

Signed-off-by: Louis Peens <louis.peens@netronome.com>
Signed-off-by: Nic Viljoen <nick.viljoen@netronome.com>