dt: bindings: add bindings for wcn3990 wifi block

Add device tree binding documentation details for wcn3990
wifi block present in Qualcomm SDM845/APQ8098 SoC into
"qcom,ath10k.txt".

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>