dt-bindings: ath10k: Add wifi-firmware subnode for wifi node

Add a wifi-firmware subnode for the wifi node.
This wifi-firmware subnode is needed for the
targets which do not support TrustZone.

Signed-off-by: Rakesh Pillai <pillair@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1586971906-20985-2-git-send-email-pillair@codeaurora.org