image-uefi: Add cmdline to use SOF firmware signed with production key

Task https://phabricator.apertis.org/T9191

The UP Squared 6000 board fails to bring up audio as it tries to load
the SOF firmware that is signed with the community key.

Starting from Linux v5.16, the following patch is included:

  commit 405e52f412b85b581899f5e1b82d25a7c8959d89 (bad)
  Author: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>

      ASoC: SOF: sof-pci-dev: use community key on all Up boards
  ---
   sound/soc/sof/sof-pci-dev.c | 10 +---------
   1 file changed, 1 insertion(+), 9 deletions(-)

Although the community key is the preferred option to allow developers
to sign their own firmware, the Skylake found in the UP Squared 6000
board only supports firmware signed with the production key.

Fix this by adding the module option (snd-sof-pci.fw_path="intel/sof")
to the kernel cmdline arguments, so the right firmware is picked up.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>