dt: bindings: net: add qcom,ath11k.yaml

ath11k is a driver for Qualcomm IEEE 802.11ax devices. Add a
bindings document for the driver, first documenting IPQ8074 which is the
only device ath11k currently supports.
Signed-off-by: Anilkumar Kolli <akolli@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>